The IR and Raman spectra of various saturated solutions of hitropyridine N-oxide (NPO) and NPO-d, were recorded in the range 4000-350 cm-I. They were assigned according to previous results for the crystal. A strong Fermi resonance is observed in the zyxwvutsrv,,(NO,) region for solutions of NPO in CCI,, CS, and C,Br,F,. The hydrogen bonds between a proton of a water or a methanol molecule and the N-oxide group of NPO are strong. These hydrogen bonds favour one of the two resonance forms of NPO and modify the spectra of the saturated solutions of NPO in donor solvents. Comparison with the vibrational frequencies of the crystal also shows that the main component of the crystalline field is a dipoldipole interaction, the strength of which is approximately that ofa hydrogen bond between NPO and a methanol or a water molecule.
zyxwvutsrqponm JOURNAL OF RAMAN SPECTROSCOPY, VOL 19, 499-502 (1988) Vibrational Study of 4-Nitropyridine N-Oxide HI*-The Charge-Transfer Phenomenon as Observed in Solution Marc Joyeux, Gerard Menard and Nguyen Quy Daot zyxwv Laboratoire de Chimie et Physico-Chimie Minerales, UA 441 du CNRS, Ecole Centrale des Arts et Manufactures, Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, France zyxwvutsr The IR and Raman spectra of various saturated solutions of hitropyridine N-oxide (NPO) and NPO-d, were recorded in the range 4000-350 cm-I They were assigned according to previous results for the crystal A strong Fermi resonance is observed in the v,,(NO,) region for solutions of NPO in CCI,, CS, and C,Br,F, The hydrogen bonds between a proton of a water or a methanol molecule and the N-oxide group of NPO are strong These hydrogen bonds favour one of the two resonance forms of NPO and modify the spectra of the saturated solutions of NPO in donor solvents Comparison with the vibrational frequencies of the crystal also shows that the main component of the crystalline field is a dipoldipole interaction, the strength of which is approximately that of a hydrogen bond between NPO and a methanol or a water molecule INTRODUCTION The molecule of 4-nitropyridine N-oxide (NPO) is very interesting both in c h e m i ~ t r y ' ~and in non-linear It has already been extensively studied by x-ray and neutron diffraction methods,'-'' but data on the vibrations of the molecule are scarce and incom~lete."-'~ To remedy this, we previously examined thoroughly the Raman spectra of the single crystal of NPO and the IR absorption spectra of a polycrystalline sample.' ',I6 This paper completes these studies and we report and assign the spectra of various saturated solutions of N P O ; the differences between these spectra enable us to obtain some insight into the intermolecular interactions between NPO and the solvent, and also into the interactions inside the crystal accumulations were often required The polarization of the incident beam was rotated by a half-wave plate to measure the depolarization ratios The IR absorption spectra of the same solution were recorded between 1600 and 1050 cm-l on a PerkinElmer 580 spectrophotometer, using compensated double-beam geometry and CaF, windows The IR absorption spectra of various other saturated solutions mol 1-' in CCl,, x mol 1-L of N P O (8 x in CS2, about mol 1-' in C,Br,F,, 0.2 mol 1- in acetone and 0.1 mol 1-1 in methanol) were recorded in the range 4000-350 cm-' on the same spectrophotometer using KBr windows The resolution was better than cm-' and the accuracy of the reading of the frequencies was +_ cm - below 2000 cm and & cm above 2000 cm-' These spectra are reproduced in Fig The IR absorption spectra of the solutions of NPO-d, were recorded using the same conditions and are reported in Fig There are many fewer discernible lines in these spectra than in the spectra of the solid samples; in particular, the absorption intensities of the out-of-plane vibrations ( A , or B, symmetry in the point group C,") are much lower than those of the in-plane vibrations ( A , or B, symmetry), and only three of the out-of-plane vibrations can be distinguished in the spectra However, in spite of this, our previous studies on the crystalI6 permit a straightforward assignment of the spectra of the solutions of N P O (Table 1) and NPO-d, (Table 2) The v,,(NO,) region displays some special features : whereas only one line is expected in this region, as in the spectra of the solutions in methanol [v,,(NO,) = 1533 cm-'1 and in water [v,,(NO,) = 1537 cm-'1, there are two lines at 1515 and 1527 (or 1528) cm-' in the spectra of the saturated solutions of N P O in CCl,, C,Br,F, and acetone Moreover, the intensity of the vs(N02) band is nearly equal to that of the v,,(NO,) band on the first two spectra, and this is also verified for the last three spectra, if we take for v,,(NO,) the sum of the intensities of the two bands that appear near 1520 zyxwvutsr zyxwvutsrq zyxw zyxwvutsrq EXPERIMENTAL AND VIBRATIONAL ASSIGNMENT Owing to the low solubility of N P O in the solutions studied, the IR spectra were much easier to record than the Raman spectra Nevertheless, we recorded the whole Raman spectrum of the solution of NPO in water and the Raman spectra in the C-H frequency range of every solution, because in these cases the IR spectra were obscured by the bands of the solvents or by the absorption of the windows The Raman spectra of a 0.2 moll-' solution of NP O in water were recorded with a Dilor RTI 30 triple monochromator, using 514.5 nm radiation of an argon ion laser for excitation (100 mW power on the sample) The resolution was better than cm-' Several tens of ' ' ' zyxwvutsrqpon zy * For Part 2, see Ref 16 t Author to whom correspondence should be addressed 03774M86/88/08049904$05.00 Q 1988 by John Wiley & Sons, Ltd Received 20 January 1988 Accepted 24 March 1988 500 zyxwv zyxwvutsrqponml M JOYEUX, G MENARD A N D N Q DAO CCI, n -7B zyxwvutsrqp I I Y CH,Q( I W A Y E NUMBER (cm-') 1 1 lo00 ~ 1 I 1 l ~ 500 Figure Difference IR absorption spectra of various saturated solutions of NPO-d, The solvent is given under each spectrum In contrast, the bonds between the oxygen atom of N P O and the OH groups of the solvents are strong ShindoI3 showed that the 0-H stretching vibration of methanol is shifted by 348 cm-' to lower frequencies when a hydrogen bond is formed between N PO and methanol This shift is equal to that due to the inter- action of a carboxylic acid, such as acetic acid, with a strong base, such as dimethyl s u l p h o ~ i d e ' This ~~~~ proves that the hydrogen bond is strong (AH x kcal mol- I), and the formula of Pimentel and Sederholmz' gives an O distance of 2.76 A The NOz group is a bad acceptor of a hydrogen bond," whereas the N-oxide group is a good acceptor.z3 Thus, the bond between NPO and methanol or water involves the N-oxide group and not the NO, group NMR spectroscopic studies having proved that a substantial contribution from resonance form I1 (Fig 3) must be associated with the hybrid which characterizes NPO,*, we conclude that the formation of hydrogen bonds between N P O and the solvent will tend to favour resonance form I, which has a large excess of electronic density on the oxygen atom in comparison with resonance form 11 This is clearly shown by the behaviour of the broad band which appears at 1303 cm-' (03) in the spectrum of the solution of NP O in CCI,, but is lowered to 1271 cm-' for the solution in methanol and to 1248 cm-' for the solution in water This band must therefore be assigned, according to the early work of Shindo," to the N-oxide stretching vibration,* the lowering of its frequency being due to the decrease in the order of the N-oxide bond All the remaining changes in the spectra induced by donor solvents (methanol and water) can also be explained with this argument The frequencies of the zyxwvutsrqpon zyxwvutsrqpo zyxwvutsr zyxwvuts zyxwvut Figure Difference IR absorption spectra of various saturated solutions of NPO The solvent is given under each spectrum cm-' This is a typical case of Fermi resonance, the only reasonable assignment of which is 030 (~ + 03( z 1300 cm- ', A , , ox)z cm-', B,,6,) 1530 cm-' (B,) This resonance is missing in the spectra of the solutions of N P O in methanol and in water and in the spectrum of the crystalline sample, because in these cases the o3 band is shifted by more than 30 cm-' to lower frequencies (see below) Similarly, the deuteration of the NPO molecule induces a 25 cm-' downwards frequency shift of the o3band, and the Fermi resonance cannot be observed in the spectra of NPO-d, INTERACTIONS BETWEEN NPO AND SOLVENTS An aromatic compound which is not substituted by at least three electronegative atoms cannot behave as a proton donor in a hydrogen bond.' * Accordingly, we observed no variation in the C-H stretching frequency of N P O in the Raman spectra of the saturated solutions of NPO in CCI, (vCH = 3103 cm-I), acetone (vCH = 104 cm - ') and water (vCH= 103 cm ') zyxwv * The figure for w g that was given by Schmid et a1." displays inphase stretchings of the two bonds between the ring and the substituents in para positions However, this figure was drawn for the case of equal substituents, and is not entirely valid in the case of NPO zy zyxwvuts zyxwv VIBRATIONAL STUDY OF CNITROPYRIDINE N-OXIDE PART I11 Table Vibrational frequencies (cm- ') of NPO in various solvents CCI, CS, Acetone MeOH Crystal (1R) (W (Iff) OR) (IR) - - - 464 500 644 464 500 644 - 465 645 647 676 752 857 870 1020 675 752 862 872 366" 466 516 647 655" 682 750 866 874 1019 1096 1124 1169/75d 1286 1271 1347 1365 1446 1463 1515 13 15 22 12 24 Y(NO2) 14 21 29 20 11 vs(N02) 28 27 19 v,,(NO2) 1588 1604 3075 3080 3101 3113 10 - - 675 674 748 847 868 1020 1084 1117 1167 1288 1304 1337 1351 850 868 1020 1085 1118 1169 1288 1303 1340 1354 1443 1467 1515e 1527" 1604 3119 - - - - 1120 1170 1288 1297 1344 1121 1174 - 1271 1347 1356 - - - 1467 1515" 1527" 1583 1604 3113 - - 1533 1592 1604 - Frequency NO.'' 50 zyxwv Descriptionb zyxwvuts The subscripts are the measured depolarization ratios out-of-plane bending; w = ring bending or stretching vibration; 6,, (or hCD)= C-H (or C-D) in-plane bending; wx = N-0 and C-NO2 stretching +ring bending or stretching vibration; v,(NO,) and v,,(NO,) = NO, symmetric and antisymmetric stretching; y(N0,) = NO, wagging; 6, = N-0 and C-NO, in-plane bending; *(NO,) = NO, bending; vCH (or vCD)= C-H (or C-D) stretching Frequency read on the Raman spectrum d s ' ' plitting due to the dynamic coupling Fermi resonance (see text) a br= Ring torsion; yCH = C-H v,(NO,) and v,,(NO,) vibrations are increased, but the shift is lower than for the N-oxide stretching band, the difference between the NO, bond orders of resonance forms I and I1 being only 0.5 This result is consistent who showed that a with that of Kross and F a ~ s e l , ' ~ weakening of the donor power of the substituent of a para-substituted aromatic ring is characterized by an increase in the NO, bonds and in the v,(NO,) and v,,(NO,) frequencies Similarly, the global electron density on the ring is higher in resonance form I than in 11 Hence, apart from the N-oxide stretching vibration 03, which also involves some stretching of the pseudo- Table Vibrational frequencies (cm- ') of NPO-d, in various solvents Frequency CCI, 451 588 623 697 835 888 1079 1264 1332 1390 1517 a CS, 450 623 696 835 886 1077 1264 1328 1389 - Acetone MeOH Crystal - 451 624 700 838 887 1080 1258 1332 1390 1517 1568 626 454 596 625 700 84C-845b 884-888b 1080 1238 1333 1384 1511 1568 2294 2330 - 840 888 1082 1233 1335 - 1525 1570 2315 See footnote b in Table Splitting due to the dynamic coupling NO.'' 13 24 22 20 Description' 502 zyxwvutsrqponml M JOYEUX, G MENARD AND N Q DAO quinonoid structure of NPO, the ring and C-H bending frequencies must be shifted to higher frequencies; this is actually what is observed in the spectra To conclude this section, we can say that the effect of the intermolecular bonds in the solutions is opposite to that of the charge transfer which takes place in this molecule; resonance form I is favoured, which means that an electronic doublet tends to be transferred from the NO, group towards the N-oxide group zyxwvuts zyxwvu zyxwvuts zyxwvu COMPARISON WITH THE CRYSTALLINE FIELD Crystallographic studiesg have shown that the crystal of NPO consists of parallel layers containing the planar molecules We reported earlier15 that there are two possible types of intermolecular contacts inside this crystal : (i) hydrogen bonds between two molecules belonging to the same layer and (ii) dipole-dipole interactions between the N-oxide group of a molecule and the NO, group of a molecule in a layer situated immediately above or below The fact that the H distances remain fairly large (at least 2.34 A) and that we observe no significant frequency shift of the C-H stretching vibrations when the crystalline sample is in solution in CCl, confirms that the hydrogen bonds in the crystal are also negligible However, the o3 and the v,(NO,) vibrations have the same frequencies in the crystal and in the solution in methanol Moreover, nine vibrations of NPO, out of the Figure The two main resonance forms of NPO eleven that are observed in the spectrum of the soluion in water, have a frequency in the crystal which is located between the frequencies of the solutions in CCl, and in water Hence, the crystalline field has an effect on the vibrations of NPO, and a strength, which are similar to that of a hydrogen bond between an NPO and a methanol or a water molecule The reason is straightforward: as with the electronic deficiency of the methanol or the water protons, the dipoles of the two NO, groups located precisely above and below the N-oxide group of a third molecule tend to favour the resonance form I of this molecule, which exhibits, along the N-oxide bond, a strong dipole in a head-to-tail configuration with those of the NO, groups zyxwvutsr zyxwvutsrq zyxwvutsrqpon REFERENCES E Ochiai and M Ishikawa, Proc Jpn Acad 18, 561 (1942) H J den Hertog and W P Combe, Red Trav Chim PaysBas70, 581 (1951) H J den Hertog, C R Kolder and W P Combe, Recl Trav Chim Pays-Bas 70,591 (1951) E Ochiai, J Org Chem 18, 534 (1 953) J L Oudar and D S Chemla, J Chem Phys 66, 2664 (1977) J L Oudar, J Chem Phys 67,446 (1 977) J Zyss, D S Chemla and J F Nicoud, J Chem Phys 74, 4800 (1981) E L Eichhorn,Acta Crystallogr 9, 787 (1956) Y Wang, R H Blessing, F K Ross and P Coppens, Acta Crystallogr.,Sect B 32, 572 (1 976) 10 F L 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A 28,2343 (1 972) 21 G C Pimentel and C H Sederholm, J Chem Phys 24, 639 (1956) 22 W F Baintinger, P von Rague Schleyer, T S S R Murty and L Robinson, Tetrahedron 1635 (1 964) 23 D Hadzi, H Ratajczak and L Sobczyk, J Chem SOC.A 48 (1967) 24 S A Sojka, F J Dinan and R Kolarczyk, J Org Chem 44, 307 (1 979) 25 R D Kross and V A Fassel, J Am Chem SOC.78, 4225 (1 956) ... the formation of hydrogen bonds between N P O and the solvent will tend to favour resonance form I, which has a large excess of electronic density on the oxygen atom in comparison with resonance... deuteration of the NPO molecule induces a 25 cm-'' downwards frequency shift of the o3 band, and the Fermi resonance cannot be observed in the spectra of NPO-d, INTERACTIONS BETWEEN NPO AND SOLVENTS An... C-NO2 stretching +ring bending or stretching vibration; v,(NO,) and v,,(NO,) = NO, symmetric and antisymmetric stretching; y (N0 ,) = NO, wagging; 6, = N- 0 and C-NO, in-plane bending; *(NO,) = NO,