Spectroscopic and theoretical studies of 4-nitropyridine N-oxide and of its related charge transfer compounds in their excited state T1

The geometries of both the singlet ground state and the lowest triplet excited state of NPO have been opti- mized through minimization of the total energy gradi- ent at the ab initio SCF level of calculation. Those optimizations have been carried out using basis sets of sizes (9s 5p) for C, N, 0 and (4s) for H [ 231, which were contracted into split-valence, that is [ 3s,2p] for C, N, 0 and [2s] for H. They will be referred to as basis sets I. Starting from the same set of primitive Gaussians for C, N, 0, but from a more extended (6s) set for H [ 241, a more flexible contraction scheme still minimal for the inner shell of C, N, 0, but triple-f for the valence shell (basis sets II), has been used for further optimization of the N-oxide distance. Finally, polarization functions have been added on the top of basis sets II in order to generate a quantitatively reliable description of the electron deformation density in both the ground state and the lowest triplet state. A p-type polarization function with exponent 0.8 has been used for hydrogen, d-type functions with exponents 0.63, 0.95 and 1.33 were used for C, N, and 0, respectively. Those polarized basis sets (basis sets III) have led to quantitative accuracy for peptide molecules when com- pared to static model distributions derived from X-ray diffraction experiments [ 251. All calculations have been carried out using the ASTERIX package [26], except for the geometry optimization of the open-shell state which has been performed using the HONDO program [ 271.

Chemical Physics ELSEVIER Chemical Physics 182 ( 1994) 313-323 Spectroscopic and theoretical studies of 4-nitropyridine N-oxide and of its related charge transfer compounds in their excited state T1 Frangoise Briffaut-Le Guiner a, Pascal Plaza a, Nguyen Quy Dao a, Marc BCnard b aLaboratoire de Chimie et Physico-Chimie Moltkxlaires, ERS 0070 CNRS, Ecole Centrale de Paris, Grande Voie des Vignes, 92295 Chritenay-Malabry, Cedex, France b Laboratoire de Chimie Quantique, Institut Le Bel, rue Blaise Pascal, 67ooO Strasbourg, France Received October 1993; in final form 25 January 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUT Abstract Lowest triplet state Tr Raman spectra of 4-nitropyridine N-oxide NPO, its deuterated derivative NPO-d., and its related compound 3-methyl 4-nitropyridine N-oxide (POM) obtained by time-resolved resonance Raman spectroscopy (TR3) are reported NPO and NPO-d, keep the CzVsymmetry in their zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE T, state but the structure of the cycle is modified towards a pronounced quinonoid conformation An important charge transfer from the N-oxide to the NO1 group is observed The TR3 activity of TrPOM vibrations is very similar to T,-NPO, the T, + T, transitions of these molecules are of the same nature Ab initio SCF calculations have been performed and the results obtained for both geometry and electron deformation density are in good agreement with spectroscopic data Introduction The molecule of 4-nitropyridine N-oxide (NPO) #l plays an important role in chemical synthesis and its related compound 3-methyl 4-nitropyridine N-oxide (POM) is used as a very efficient non-linear optical (NLO) medium NPO is the main chemical intermediate for the preparation of 4-pyridine substituted derivatives and of pyridine N-oxide [2], and has an excellent photochemical reactivity Unlike most aromatic oxide amines, its reactive site is not situated on the N-oxide group but on the nitro group 13-51 It has been shown [ 61 that photochemical reactions of NPO are initiated by its T, state in desoxygenated water and * Corresponding author IT’Preceding paper of the series; ref [ 11 0301-0104/94/$07.00 1994 Elsevier Science B.V All rights reserved SSDIO301-0104 (94)00050-K by its S, state in alcohols NLO properties have been demonstrated for POM, which has proved through various experiences to qualify as a highly efficient quadratic NLO material in the visible and near-IR range (from 0.5 to 1.8 pm) [ 7-91 This property can be explained by an internal charge transfer (ICT) phenomenon occurring in the singlet excited state (S,) between the N-oxide donor group and the nitro acceptor group, through the aromatic ring In order to quantify the ability of these compounds to undergo solvent-induced ICT in their ground state, vibrational spectroscopic studies of those two molecules have been done in this laboratory using both experimental and semi-empirical methods [ l,lO-151 As far as ICT in the Si state is concerned, it seems that the extremely short lifetime of this state [ 161 ( < 100 fs) prevents easy spectroscopic investigation Never- 314 F Brijtiut- Le Guiner et al /Chemical Phy sics 182 (1994) 313- 323 theless the long-lived T, excited state of NPO has been shown to exhibit also an ICT character [ 171 The aim of the present work is to study this ICT character of the T, state, for NPO and POM, by time-resolved resonance Raman (TR3) spectroscopy and ab initio calculations Experimental I Chemicals NPO was purified by recrystallisation of NPO (Merck 98%) NPO-d, was synthetised from pyridined5 (99.8%) using Ochiai procedure [ 181 Pure POM single crystals were prepared by Zyss and co-workers [ 191 These compounds were dissolved in bidistilled water at the concentration of lo-’ M for TR3 experiments 2.2 The TR3 setup The S, state (IT-~* of ‘Ai symmetry) of NPO has a strong absorption band in the UV range and the T, state (n-n* of 3A, symmetry) has a strong and broad T, -+T, absorption band in the visible range (maximum absorption at 550 nm) [ 71 The T, -+ T, absorption of POM is still unknown, but owing to the similarity of structures, it is expected in the same region as for NPO It is possible under these conditions to perform time-resolved “pump-probe” resonance Raman experiments using a single pulsed laser to study the T, state of these compounds The T, states of NPO, NPO-d, and POM were populated by the third harmonic (355 nm, IO ns duration) of a Q-switched Nd : YAG Laser (Quanta Ray, model GCR 4,10 Hz), in the S, -+ S, absorption band (whose maxima in water lie respectively at 14 and 307 nm for NPO and POM) followed by an ultra-fast intersystem conversion S, -+ T, Energy of mJ/pulse is used for the experiments in order to avoid multiple absorption [20] For the generation of the Raman exciting line, the second harmonic (532 nm, 10 ns duration) of the same laser was used to pump a home-made Littmantype dye laser [ 21,221 The dye solution were rhodamine 575 (Exciton), rhodamine 610 (Exciton) and DCM (Exciton) in ethanol for tunable excitation between 550 and 620 nm The probe beam intensity was fixed at 3.5 mJ/pulse at the level of the sample The probe beam was delayed by 10-15 ns with regard to the pump beam in an optical delay line and then focalized colinearly in a capillary tube where the sample solution was circulating The transit time of the solution in the pumped volume was about lo-” s so that solution was refreshed before each laser pulse The Raman spectrum was collected at 90” with the help of a triple monochromator Raman spectrometer (Jobin-Yvon S3000) equipped with a 700 intensified photodiodes multichannel detector The spectra were recorded between 500 and 2000 cm-’ accumulating 100 spectra of 10 s integration time each, which corresponds to 10000 pulses per spectrum 2.3 Ab initio calculations: computational details The geometries of both the singlet ground state and the lowest triplet excited state of NPO have been optimized through minimization of the total energy gradient at the ab initio SCF level of calculation Those optimizations have been carried out using basis sets of sizes (9s 5p) for C, N, and (4s) for H [ 231, which were contracted into split-valence, that is [ 3s,2p] for C, N, and [2s] for H They will be referred to as basis sets I Starting from the same set of primitive Gaussians for C, N, 0, but from a more extended (6s) set for H [ 241, a more flexible contraction scheme still minimal for the inner shell of C, N, 0, but triple-f for the valence shell (basis sets II), has been used for further optimization of the N-oxide distance Finally, polarization functions have been added on the top of basis sets II in order to generate a quantitatively reliable description of the electron deformation density in both the ground state and the lowest triplet state A p-type polarization function with exponent 0.8 has been used for hydrogen, d-type functions with exponents 0.63, 0.95 and 1.33 were used for C, N, and 0, respectively Those polarized basis sets (basis sets III) have led to quantitative accuracy for peptide molecules when compared to static model distributions derived from X-ray diffraction experiments [ 251 All calculations have been carried out using the ASTERIX package [26], except for the geometry optimization of the open-shell state which has been performed using the HONDO program [ 271 F Br$aut-Le Guiner et al /Chemical Physics I82 (1994) 313-323 315 effectively created by a one-photon process as expected (iii) The scaling factor gives an estimation of the depopulation ratio of the So state For a pump beam of mJlpulse in power, it is found to be equal to 10% for both NPO and POM (iv) All the spectra were recorded with 0.01 M solutions of NPO, NPO-d, and POM Several wavelengths, around 550 nm, corresponding to the maximum position of the T, + T, absorption band, were used in order to check the intensity variation of the Raman lines at resonance, pre-resonance and off-resonance wavelengths Figs 3a and 3b show the Raman enhancement profiles of the strongest peak of the transient species, which lie at 982 and 940 cm-’ respectively for NPO and POM This peak is about 50 times stronger for the 550 nm + resonance excitation than for the 620 off-resonance 1400 ‘200 1004 excitation in the case of NPO The Raman enhancement Wavenumbers (cm-‘) profile is the same as for the Tr -+ T, absorption band, Fig Raman spectra of POM recorded with a 550 nm excitation suggesting that the transient species observed on the (A) With a pump beam at 355 nm (B) Without the pump beam resonance Raman spectra is the T, state of NPO Fur(C) The transient spectrum obtained using C = A - ( 1lk)B zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG ther proof of the observation of the T, state by oxygen T Intensity (arbitrary unit) Results Spectra of NPO, NPO-d, and POM in aqueous solution obtained without the pump beam show as expected only Raman lines of the So state The ones recorded with the pump beam show extra lines, due to the transient species The spectra of the transient species were obtained by difference and then scaled, as shown in Fig for POM Fig gives the Raman spectra of NPO, NPO-d, and POM in their excited state T, Before going to the assignment of the Raman lines, some preliminary remarks must be drawn: (i) Raman spectra of low3 M solutions of NPO and POM were first recorded with 550 nm excitation Without the pump beam, no Raman band was observed, the concentration of the molecules in the So state is too low to be detected With the pump beam, Raman peaks appear for both compounds The number and positions coincide with the transient spectra obtained with lo-’ M solutions These results show that the resonance Raman phenomenon occurs for both NPO and POM (ii) The Raman intensity of the transient species varies linearly with the pump beam intensity around ml/pulse energy, which means that this specifies is I ! 400 600 800 1000 ,200 1400 Wavenumbers l (cm-‘) Fig Ramaa spectra in their T, state of (A) NPO, (B) NPO-d, and (C) POM F Bri@ tr-Le Guiner et al / Chemicul Phy sics 182 (1994) 313- 323 zyxwvutsrqponmlkjihgfedcbaZYXW 316 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA , i(l’,) (arbitrary linear unit) the same as in T,-NPO The two extra peaks at 1026 and 1420 cm-’ can be assigned without any doubt respectively to p(CH,) and s(CH3) They are only slightly shifted when passing from S, to T, states ( 1038 and 1417 cm- ’ in S,,-POM crystal) A certain number of the Raman lines on the three spectra can be assigned unambiguousIy either because of their comparable intensity, or due to the isotopic 580 560 600 620 Wavelength (nm) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA effects for NPO and NPO-d,, or by comparison with the spectra of the S,, states The two very strong peaks at [ 1631, 1589, 1621 cm-‘] and [ 1434, 1368, 1401 cm-‘] for respectively [ NPO, NPO-I-I,, POM] can be assigned to the C-C or C-N stretching ring modes 8a and 19a (Wilson’s notation for aromatic ring) They are observed at [ 1606, 1573, 1611 cm-‘] and [ 1476, 1388, 1478 cm-‘] for 580 860 540 600 620 the S,) states in aqueous solutions Wavelength (nmf The two medium peaks at [ 1335,1325,1322 cm-‘] Fig Kaman enhancement profile of the T, strongest Raman peak and at [ 835,822,730 cm- ‘1 are assigned respectively respectively at 982 cm-’ for (A) NPO and at 940 cm-’ for (B) to v,(NO,) and mode I (breathing of the cycle) They POM appear at [ 1359, 1341, 1353 cm-‘] and at [ 875, 849, (no.) cm- ’ ] in the ground state It is to be noted that quenching could not be achieved, due to the low solumode is not observed f n.o.) for POM in the S, state bility of oxygen in water and to the fixed pump-probe The weak and very weak peaks at [ 1289,12 lo,1302 delay time, = 15 ns, which does not allow the obsercm ‘], [1098, 1072, 1085 cni-‘I, [631, 612, 615 vation of the decay kinetics cm - ’ ] and [ 368,362,368 cm ~ ’ ] are assigned respecFor POM, the T, ) 1;, absorption spectrum was still tively to modes 7a, 13, 12 and 6a These modes result unknown but according to the resonance Raman from a coupling of the in plane (ip) cycle deformation enhancement profile, its maximum can be located and the stretching vibrations of the substituents They approximately at 560 nm zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA appear respectively at [ 1248, 1208, 1296 cm-‘], [1125, 1084,1098cm”“], [647,631,647cm-i] and Assignment [ 362, 366, 366 cm _ ’ ] in the ground state So In order to make the assignment of the bands, folThe in-plane deformation vibrations of NFO and lowing the Franck-Condon principle [28] for resoPOM are expected between 1000 and 1300 cm-’ and nance Raman spectra, the enhanced modes must belong shifted to 700-1000 cm-’ ’ for NPO-& The very strong to the A, symmetry of a common point subgroup shared peaks at 982 cm- ’ for T,-NPO and 940 cm-’ for Tiby the molecular geometries of the T, and T,, states If POM can be assigned to the f8a mode: they appear at both states have CzV symmetry, which means that the 103 and 1032 cm ’ respectively for S,-NPO and Somolecule retains its planar configuration in T, and T,, POM A comparison of the frequency shifts of 18a for one should expect I observable A, modes If the symthe St, and T, states of those two molecules suggests metry is lowered, for example if the NO, group is that the corresponding mode of T,-NPO-d, should twisted, the symmetry of the molecule would be C, and appear around 730 cm- I However, it is not observed there woufd be 18 observable A, modes ExperimenIt is to be noted that this mode is not observed either in tally, there are only 11 observed frequencies in the TR3 the Raman spectrum of So-NPO-d, but only in its IR spectrum of NPO, 10 frequencies and a shoulder for spectrum [ 13,141 The 9a mode is present in the three NPO-d, Therefore, the symmetry of Ti-NPO is CzVas spectra at 1158 cm-’ for Ti-NPO, 888 cm-’ for T,it is for S,-NPO NPO-d, and 1144 cm- ’ for T,-POM The assignment The T,-POM spectrum exhibits 13 Raman peaks is based only on the comparison of the frequency shifts, including two unresolved doublets Its general feature since the intensities greatly vary from one compound is F Briffaut-Le Guiner et al /Chemical Physics 182 (I 994) 313-323 317 Table Tl state Raman frequencies (cm- ‘) of NPO, NPO-d, and POM and their assignment The corresponding frequencies at the So state of these compounds in aqueous solution are given for comparison (values in brackets are only observed in the crystalline state) NPO NPO-d, T, SO” 368 (w) 631 (VW) 767 (w) 835 (m) 982 (s) 1098 (w) 1158 (w) 1289 (w) 1335 (m) 1434 (s) 1631 (s) 362 (647) (858) 875 1031 1125 1180 1248 1359 1476 1606 T, 362 (w) 612 (VW) 809 (m) 822 (sh) 1072 (w) 888 (m) 1210 (w) 1325 (m) 1368 (s) 1589 (s) Assignment POM Sob T1 So’ notation description * (366) 631 857 849 (772) 1084 892 1208 1341 1388 1573 368 (w) 615 (VW) 789 (m) 730 (m) 940 (s) 1085 (m) 1144 (s) 1302 (m) 1322 (m) 1401 (s) 1621 (s) 1026 (w) 1420 (m) (366) (647) (848) no (1032) 1098 1179 1296 1353 1478 1611 (1038) (1417) 6a 12 W :N02) 18a 13 9a 7a 19a 8a so_n(Sc_o) L( Lo) 0, v,(ND,) 0 p(CH,) %,(CHs) “Ref [24] b Frequencies observed and assigned in this work (IR and Raman spectra of NPO-d, in zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON aqueous solution were not recorded before) ‘Seeref [35] d o: in-plane cycle deformation; ox in-plane cycle deformation and substituents stretching; S,, (S,,) : in-plane deformation of C-H (C-D) bonds; v,( NO*) : symmetrical stretching of the nitro group; p( CH,) : rocking of the methyl group; (CH,) : deformation of the methyl group to the other: weak for NPO, medium for NPO-d, and strong for POM These ten above-mentioned frequencies observed for the Tr state were assigned without difficulty to the totally symmetrical mode of the molecules possessing the CZVsymmetry The last peak of the T, state still remains unassigned: the peak situated at 767 cm- ’ ( w) for T,-NPO, at 809 cm-’ (m) for T,-NPO-d, and 789 cm-’ (m) for Tr-POM There are only two possible assignments: the first one would be to assign it to y (NO,) an out-of-plane (op) vibration which appears at [752,700,753 cm-‘] [ 13-171 in the S,, state But since no other op deformation modes were observed, neither the op CH deformations nor the cycle op bending modes, this assignment would be very unlikely The other possible assignment is the symmetricaldeformation 6,(NO,) mode of the NO, group which is the last totally symmetrical mode of the CZVsymmetry, also expected in this region Such an assignment would make this mode the most perturbed one when passing from Se to Tr : the frequency is lowered by cm- ’ for NPO, by 48 cm- ’ for NPO-d,, and by 59 cm-’ for POM This important frequency shift will be discussed in the next section The assignment of the Raman peaks is summarized in Table Discussion 5.1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO The cases of NPO and NPO-d, Only the vibration modes are totally symmetrical under the constraints of the CZV point group are enhanced for T,-NPO and T,-NPO-d4 As already stated, these molecules retain their CZVsymmetry in the T, states Moreover, the enhanced modes are not only active on a specific molecular fragment but are delocalized on the whole molecule However, the strongest intensities are the cycle vibrations (8a, 19a, 18a), which basically locate the T, + T, chromophore on the cycle Although modifications in the description of the normal modes are unavoidable when passing from the So to the T, state, we will base our discussion about the frequency shifts on the results of the normal coordinate analysis (NCA) previously performed for NPO and NPO-d, in their ground states [ 161 The ip deformation modes 1,8a and 19a are modified by the promotion of one electron from the bonding v orbital to the antibonding n* orbital: (i) The important lowering of the cycle breathing frequency (A V= v(T,) - v(T,) = - 40 and - 27 cm-‘) shows that, as for the benzene [ 291 and toluene F Briflaut-Le Guiner et al /Chemical 318 Physics 182 (1994) 313-323 and 18a modes While the NCA results [ 151 showed an important contribution of the C-C stretching vibrations of the bonds parallel to the molecular axis for the 9a mode, on the contrary it showed for the 18a mode contribution of the C-C stretching vibrations of the 18a 9a v.(NW bonds non-parallel to this axis (Fig 4) This can explain why the frequency shift is less important for 9a , (Av= -22 cm-‘) than for 18a (Av= -49 cm-‘) , between the S,-NPO and T,-NPO states Concerning NPO-d,, it is essentially the C-C stretching vibrations of the bonds non-parallel to the molecular axis, in addition to the deformation vibrations 6CD, who contribute I zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB I to the 9a mode; this is the reason why the frequency 12 13 la 6a shift is more important for NPO-d, than for NPO Fig Some vibrational normal modes of NPO (ref [ 151) The The u,(N02) vibration frequency mode is practiatomic motions of C N and of H atoms are represented respeccally the same for NPO and NPO-$ and is mainly due tively at the scale 10 and (the atomic normalized coordinates are to the symmetrical stretching of the NO bonds (Fig taken as unity) 4) The modes leading to the deformation of NO, and to the stretching C-NO2 bonds also give a small contribution The frequency shift for u,(NO,) mode is nearly the same for NPO and NPO-d, ( A Y= - 24 and - 16 cm- ’ ) and corresponds to a loss of charge density on the NO, bonds Normal modes 7a, 13, 12 and 6a are more complex and difficult to analyze Several coordinates implying both cycle and substituent motions contribute to those modes and the effect on the frequency shifts can be oopposite (Fig 4) Only mode 7a is simple to describe as it mainly originates in the N-O bond of the N-oxide II I group, at least in the case of NPO The increase of the Fig Two resonant forms of NPO at the ground state frequency of this mode (A Y= + cm- ’ ) then cor[30] molecules, this transition induces a decrease of responds to a significant increase of the N-O bond order the global charge on the cycle in the T, state It is worth noting that the frequency (ii) In S,-NPO and S,-NPO-d4, 8a is due mainly to shift of this mode in NPO-d, is very small (A V= the stretching vibration of the C-C bonds parallel to cm- ’ ) This can be explained by the fact that the N-O stretching vibration contributes much less to 7a in this the axis of the molecule This mode is very sensitive to molecule than in NPO Moreover the deformation the quinonoid distorsion of the cycle The increase of the frequency (A Y= 25 and 16 cm-‘) for NPO-d, motions of the cycle angles also contribute to this mode when passing from So to T, means that there is a reinand tend to lower its frequency as the global charge decreases forced quinonoid ring structure Malar and Jug [31] reached the same conclusion for the pNA molecule The frequency lowering and especially of the (iii) This is also confirmed by the frequency 6(NO,) mode, and to some extent, of v,(NO,) mode decrease of the 19a mode (A Y= - 42 and - 20 cm ’ ) : clearly imply that the N-O bond order of the NO, group this mode is due to the C-C ring bonds which are not diminishes in the T, state, and that charges are more parallel to the axis of the molecule localized on the oxygen atoms The analysis of the frequency shifts between the S, The lowering of the global charge of the cycle also implies a weakening of the ip deformation of the C-H and T, states unambiguously indicates that the electronic structure of the T, state is derived from the Mulbonds and consequently a frequency decrease of the 9a $ * :‘I; ,& F Briffuur-Le Guiner etul /Chemical Ii n Physics 182 (1994) 313-323 319 solvents that form I is largely predominant [ 141 (fig 5) Fig displays the resonant forms that are most appropriate to describe the triplet state of NPO and NPO-d, 5.2 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON The case of POM H The great similarity of T,-POM and T,-NPO spectra suggests that the T, + T, transitions are quite comparable in the two molecules The resonant activity of the two methyl vibrations v(CH,) and S,(CH3) appears quite surprising This means that the motion of the Ii N H methyl substituent should be coupled with that of the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA atoms implied in the T, + T,, transition process However, these two modes in the T, state are practically not shifted with respect to the So state, thus suggesting that Fig A major resonant form of NPO at the T, state The arrows they are insensitive to the m + 7~*electronic transition represent unpaired electrons The frequency decrease of the C-H deformation modes 9a and 18a is quite similar to what was observed for NPO Mode was not observed in the So state but was calculated by NCA to show up at 793 cm- ’ [ 321 This frequency is also expected to decrease when the molecule is promoted to the T, state in conformity with the behaviour characteristic of a diminution of the global charge on the cycle The important decrease of the 19a mode frequency, which is composed of the stretching vibrations of the cycle bonds zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP 1,2,5 suggests a diminution of the charge along those bonds On the opposite way, the slight increase in frequency of the 8a mode, mainly implying the cycle bonds 2, and 5, suggests a charge increase along bond (Fig 7) Concerning the two last cycle bonds (3 and 6)) no straightforward conclusion can be Fig Numbering of the cycle bonds of POM zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA deduced from the rest of the Raman spectrum of T,POM As for NPO, the frequency lowering of the v,( NO& and 6( NOz) modes can be interpreted in terms of a loss of the electronic density along the N-O bonds of the nitro group The potential energy distribution of mode 7a in the So [ 321 state shows equal contributions from the N-oxide stretching vibration and from the N-O stretching vibrations of the nitro group The increase in 0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA frequency of this mode indicates a strengthening of the B A N-oxide bond in the T,-POM The various contributions to the modes w,, 13, 12 and 6a, namely the ip Fig Major resonant forms of POM at its T, state deformation of the cycle and of V( N-O) and u( NOa) have opposite effects on the frequency shifts which liken resonant form II of Fig This is at variance makes unreliable any attempt to interpret them from the So state for which it had been shown in polar I + II I 320 F Bnffaut-Le Guiner et al /Chemical Physics I82 (1994) 313-323 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR lone pair peaks which are as usual much sharper in the computed maps, the height of the density peaks dispIayed in the experimental model map is reproduced within an accuracy of = 10% Results of similarquality have been obtained for Leu-enkephalin [25] and for other polypeptides [25] using the same atomic basis sets It should be noted that the electron accumulation I 326 along the N-O bond is strongly displaced toward the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA N atom in both the experimental and the theoretical maps SO Fig The optimized geometry of the ground state and of the lowest It is therefore of interest to correlate the spectrotriplet state of NPO as obtained from ab initio SCF calculations scopic results obtained for NPO in the S, and T, states with the changes in the molecular geometry and in the As for NPO, trends for the structure of the T,-POM density distribution evidenced from ab initio calculacan be proposed from this analysis Figs 8(A) and tions carried out on those two states The deformation 8(B) represent the two most probable structures, but of the cycle toward a quinonoid structure in the triplet unlike T,-NPO for which the cycle has the quinonoid state is illustrated by the shortening of the C-C bonds conformation, in Tr-POM only bond is strengthened parallel to the symmetry axis and the concomitant while bonds 1, and are weakened Those results lengthening of the four other bonds of the cycle (Fig suggest that resonance form B probably competes with 9) the quinonoid form A The only variation of interatomic distance which does not seem to be in straightforward agreement with 5.3 Comparison with ab initio calculations the previous interpretation of the spectroscopic results is the N-oxide distance Even though the Raman spectra indicate that the bond order is higher in Tr than in The geometries optimized for the ground state and for the lowest singlet state (IT f n*) of the NPO molSo, the calculated N-O distance is longer in T, by 0.01 ecule are displayed in Fig It should be noted that the A However, the change in the equilibrium interatomic molecule remains planar in the T, state A comparison distance should account not only for the evolution of between the structure computed for the ground state the bond order, but also for the variation of the electroand the geometry observed by Coppens and Lehmann static interaction According to theMulliken population from neutron diffraction data [ 331 is displayed in Fig analysis, this interaction is globally attractive in the So 10 The geometry optimizations have been carried out state (computed point charges: + 0.10 e for N, - 0.59 with basis sets I For the ground state, an excellent 0 n zyxwvutsrqponmlkjihgfedcbaZYXWV agreement with the experimental results has been obtained, except for the N-O bond length, which was N computed to be 1.333 A, longer by 0.036 A, longer by 0.036 A than the experimental distance Calculations carried out with the more flexible basis sets II decreased the N-O distance to 1.326 A, and further decrease should be expected from the influence of polarization functions Apart from the N-O distance, the maximum 1.297 difference between experimental and calculated geom1.326 etries is 0.017 A,for the distances and 0.8’ for the angles 0 (Fig 9) The deformation density distribution computed A B using the polarized basis sets III (Figs 11 and 12) can Fig 10 The geometry of the ground state of NPO after theoretical also be compared with multipole model maps derived calculations (A) and as observed by neutron diffraction (B) (ref from the X-ray measurements Except for the oxygen [331) I> “Y%;5 F Bri@ ~ut-Le Guiner et (11./Chemiml ,, I I -s.aoI,,,,,,,,,,,~,,,;~~~‘~-~~~~~~~~~~~~~~~,~~~~~~~~I,~~~~~~~~~,~~~~~:~~~,~~I~~~~~~i 2.00 4.00 -2.00 0.00 -6.00 -4.00 Y 321 Phy sics 182 (1994) 313- 323 6.00 6.00 (a.u.) Fig 11 Deformation density maps for NPO obtained as the difference between the molecular density distribution and the density of a superposition of spherical atoms, both obtained from ab initio SCF calculations (basis sets III) Zero contour bold, negative contours dashed Contour interval: 0.05 e A-“ (A) molecular plane, singlet ground state So (B) molecular plane, lowest triplet state T, zyxwvut F Brijfuut- Le Guiner et al /Chemical Phy sics 182 (1994) 313- 323 322 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Fig 12 Deformation density maps for NPO, defined as in Fig 10 (A) plane perpendicular axis; singlet ground state So (B) same plane as in A; lowest triplet state T, e for 0)) but becomes repulsive in the T, state ( - 0.09 e for N, - 0.18 e for 0) This modification of the electrostatic balance should be attributed to the important charge transfer occurring from the IT orbital of oxygen to the 7~orbital of nitrogen and eventually leading to an interchange between the regions of density accumulation and depopulation along the IT system of the N-oxide bond (Fig 12) Simultaneously, the N-O u bond acquires in the triplet state more covalent character, as illustrated by the displacement of the bond accumulation from the nitrogen side to the center of the bond (Fig 11) Quite logically, this strengthening of the covalent character of the N-O o bond has an influence on the curvature of the potential well near the equilibrium Calculations carried out for several N-O to the molecular plane and containing the symmetry distances in the vicinity of the equilibrium position in the So and T, states show that the potential well is significantly sharper in the triplet state Assuming the potential energy curve to remain strictly parabolic, a 0.1 bohr deviation of the N-O distance with respect to the equilibrium position would correspond to an energy destabilization of 2.59 X lo-’ hartree for T, compared to 1.84 X 10m3 hartree for S, This computed change in the curvature of the potential well associated with the N-O stretching is in agreement with the observed variation of the associated Raman frequency, in spite of the N-O distance being slightly longer in the triplet state F Briffaut- Le Guiner et al /Chemical Phy sics 182 (1994) 313- 323 Conclusion 323 [9] I Ledoux, J Badan, J Zyss, A Migus, D Hulin, J Etchepare, G Grillou and A Antonetti, J Opt Sot Am B (6) (1987) 987 The TR3 spectra of NPO and IWO-d,, in their T, state [ 101 M Joyeux, M Jouan and Nguyen Quy Dao, J Raman Spectry have been reported and suggest that these molecules 19 (1988) 133 keep the CZVsymmetry The presence of an electron in 111I M Joyeux and Nguyen Quy Dao, J Raman Spectry 19 ( 1988) the lowest rr* orbital modifies the structure of the cycle 441 toward a pronounced quinonoid conformation This [ 121 M Joyeux G MCnard and Nguyen Quy Dao, J Raman electronic transition also induces an important charge Spectry 19 (1988) 499 1131 M Joyeux and Nguyen Quy Dao, Spectrochim Acta 44A transfer from the N-oxide to the NO2 group The chem(1988) 1447 ical photo-reactivity of the nitro group of these com[ 141 M Joyeux, M.T.C Martins Costa, D Rinaldi and Nguyen Quy pounds in their T, state in water can be explained by a Dao, Spectrochim Acta 45A (1989) 967 high charge density localized on the oxygen atoms [ 151 P Plaza, F Briffaut-Le Guiner, M Joyeux, Nguyen Quy Dao, The TR3 activity of Ti-POM vibrations is very simJ Zyss and R Hierle, J Mol Struct 247 (1991) 363 ilar to T,-NPO: the T, ) T,, transitions of these mole[ 161 M Pierre, P.L Baldeck, D Block, R Georges, H.P Trommsdorff and J Zyss, Chem Phys 156 (1991) 103 cules are of the same nature The S, -+ T, transition has [ 171 M Yamakawa, T Kubota, K Ezumi, Y Mizuno, Spectrochim also induced in POM a charge transfer from the NActa 80 (1974) 2103 oxide to the NO2 group The electronic structure of the [ 181 E Ochiai, Aromatic amine oxides (Elsevier Amsterdam, cycle in the T, state probably implies the contribution 1967) of several resonance forms, resulting from the conju[ 191 R Hierle, J Badan and J Zyss, I Cryst Growth 69 (1974) gation effect between the unpaired electron of the car545; P Andreaxxa, F Lefaucheux, M.C Robert, D Josse and J bon atom with the n electrons of the cycle Zyss, J Appl Phys 68 (1990) In order to take into account and quantitatively esti[20] N.J Frigo, IEEE J Quantum Electron QE-19 (1983) mate the charge transfer phenomenon in its globality, 1211 M.G Littman, Opt Letters (1978) it is necessary to determine the force field and the nor[22] M.G Littman and H.J Metcalf, Appl Opt 17 ( 1978) 14 mal modes of vibration in NPO, NPO-d, and POM in [23] S Huzinaga, Technical Report, University of Alberta, their T, state NCA using the geometry of the molecules Edmonton, Canada (1971) [24] S Husinaga, J Chem Phys 42 (1965) 1293 issued from ab initio calculations are now in progress [25] M Souhassou, C Lecomte, N.-E Ghermani, M.-M Rohmcr, in this laboratory [ 341 R Wiest, M Benard and R.H Blessing, J Am Chem Sot 114 (1992) 2371; V Pichon-Pesme, C Lecomte, R Wiest and M Benard, J Am References Chem Sot 114 (1992) 2713 [26] R Emenwein, M.-M Rohmer and M Bemud, Computer Phys 111D BougeW J.R Burie, Nguyen Quy Dao and B Hennion, Commun 58 (1990) 305; Spectrochim Acta, in press M.-M Rohmer, J Demuynck, M BQnard, R Wiest, C 121H.J Den Hertogand W.P Combe, Rec Trav Chim Pays-Bas, Bachmann, C Henriet and R Emenwein, Computer Phys 70 (1951) 581; Commun 60 (1990) 127; H.J Den Hertog, C.R Kolder and W.P Combe, Rec zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC T m v R Wiest and M Sp&i, PANORAMIX, Graphical facilities for Chim 70 (1951) 591 ASTERIX, unpublished results 131 G.C Spence, E.C Taylor and Buchardt, Chem Rev 70 [ 271 M Dupuis, J.D Watts, H.O Villar and G.J.B Hurst, HONDO (1970) 231 7.0, Internal Report, IBM Corp (1987) [4] C Kaneko, S Yamada, Yokoe and T Kubota, Tetrahedron Letters (1970) 2333 [28] R Clark, Vibronic processes in inorganic chemistry, ed C.D [5] N Hata, E Okutsu and I Tanaka, Bull Chem Sot Japan 41 Flint (Kluwer, Dordrecht, 1989) ( 1968) 1769; [29] G.C Nieman, J Chem Phys 50 (1969) 1674 N Hata, Ono and T Tsuchiya, Bull Chem Sot Japan 45 [30] D.M Haaland and CC Nieman, J Chem Phys 59 ( 1973) ( 1972) 2386; 4435 Ono, N Hata, Bull Chem Sot Japan 45 (1972) 295 [31] E.J.P Malar and K Jug, J Phys Chem 89 (1985) 5235 [6] N Hata, I Ono and K Osaka, Bull Chem Sot Japan 46 [32] F Briffaut-Le Guiner, Nguyen Quy Dao, M Jouau and P (1973) 3363 Plaza, Spectrochim Acta A, in press [7] J Zyss, D.S Chemla and J.F Nicoud, J Chem Phys 74 1331 P Coppens and M.S Lehmann, ActaCryst 832 (1976) 1777 (1981) 4800 [ 341 M Btnard, F Briffaut-Le Guiner, Nguyen Quy Dao, M Jouan 181 J Zyss, I Ledoux, R Hierle, R.K Raj and J.L Oudar, IEEE J and P Plaza, in preparation Quantum Electron QE-15 (1985) 1286 [ 351 F Briffaut-Le Guiner, These de I’Ecole Centrale Paris ( 1992)  ... pre-resonance and off-resonance wavelengths Figs 3a and 3b show the Raman enhancement profiles of the strongest peak of the transient species, which lie at 982 and 940 cm-’ respectively for NPO and POM... the experimental and the theoretical maps SO Fig The optimized geometry of the ground state and of the lowest It is therefore of interest to correlate the spectrotriplet state of NPO as obtained... enhancement profile of the T, strongest Raman peak and at [ 835,822,730 cm- ‘1 are assigned respectively respectively at 982 cm-’ for (A) NPO and at 940 cm-’ for (B) to v,(NO,) and mode I (breathing of