1622
J. Phys. Chem. 1983, 87, 1622-1625
Phase Effects on Conformational Equilibria. Nuclear Magnetic Resonance Studies of Methyl Nitrite J. Paul Chauvel, Jr., and Nancy S. True' Bpartment of Chemistry, Universi?. of California, Davis, Caiifornia 956 16 (Received September 15, 1982; In Fhai Form: December 13, 1982)
Gas-phase 'H NMR spectra of methyl nitrite are consistent with the following thermodynamic parameters for the syn * anti conformational equilibrium: 998 (50) cal mol-'; AGmti-syn, 520 (5) cal mol-' (at 205 K); and A&-, 2.3 (3) eu. These results agree well with values obtained from a statistical mechanical calculation. The large entropy difference between the conformers is due to a very low methyl top internal rotation barrier for the anti conformer. Neat liquid methyl nitrite and 1% solutions of methyl nitrite in carbon disulfide, acetone-d,, and n-pentane all produce temperature-dependent NMR spectra which are consistent with the following ranges of thermodynamic parameters: AHmti-syn,803-866 cal mol-'; AGmti-syn, 440-460 cal mol-' (at 205 K); ASmti-sun, 1.8-2.0 eu, demonstrating that in liquids the antifsyn partition function ratio is smaller and the equilibrium constant between the conformers is closer to 1.
Introduction This study addresses the conformer equilibrium in methyl nitrite and its dependence on environmental factors. Although all recent experimental and theoretical studies of methyl nitrite consistently demonstrate that the syn form is the most stable, reported conformer relative energy differences do not agree, and most measurements have not involved condensed-phase samples. The energy difference between syn- and anti-methyl nitrite in their vibrational ground states, based on relative intensity measurements of rotational transitions, is 314 (22) cm-'.' An enthalpy difference of 217 cm-' was obtained from relative intensities of syn/anti pairs of infrared absorption bands of methyl nitrite samples obtained by trapping thermal effusive molecular beams with various source temperatures in argon matricese2 The most recent lisuid-phase NMR study obtained a conformer relative enthalpy difference close to 0.3 Ab initio calculations yield conformational energy differences ranging from 4514 to 1030 cm-1.2 When one corrects for the sizable differences in zero-point energies for the conformers, the associated calculated ground-state anti-syn energy differences range from 350 to 929 cm-', r e s p e c t i ~ e l y . Experimental ~?~ free energy and entropy differences separating the conformers have not been reported. Recent infrared" and microwave'&spectroscopicstudies of methyl nitrite (ONOCHJ have provided a detailed picture of the energy levels of both the syn and anti conformers. An interesting difference between the two conformers is the height of the methyl top internal rotation barrier, 734 (2) cm-' (ref 1)for syn-methyl nitrite and only
10.1 cm-' (ref 8) for anti-methyl nitrite, as determined from rotational A-E level splittings. The lower barrier and associated higher state density for anti-methyl nitrite result in a vibrational partition function for this conformer which is approximately 4 times larger than that of the syn conformer. Recent ab initio Hartree-Fock molecular orbital calculations have reproduced these surprisingly large difference^.^!^ Cordell, Boggs, and Skancke have explained that the low methyl top torsional barrier in anti-methyl nitrite is largely due to a strong stabilizing attractive interaction between a C-H bond and the nitrogen lone pair when the CH and N groups are eclipsed, which reduces the height of the barrier maximum. The present study compares experimental results for the methyl nitrite conformational equilibrium to statistical mechanical calculations and various solvation models. For both gaseous and liquid samples, NMR spectra in the slow-exchange region where syn and anti resonances are cleanly resolved are experimentally accessible. Since actual conformer population ratios can be determined from NMR spectral intensity measurements, a complete set of thermodynamic parameters characterizing the equilibria can be obtained. Previous microwave and infrared studies of conformational equilibria determined the ground-state energy and enthalpy differences,respectively. The present study determines equilibrium parameters for methyl nitrite gas, neat liquid methyl nitrite, and 1Yi solutions of methyl nitrite in acetone-d,, carbon disulfide, and n-pentane. This study provides indirect information about the phase dependence of low-frequency torsional vibrations since the relative magnitude of the conformers' partition functions can be determined experimentally, and low-frequency vibrations contribute significantly to this quantity.
(1)P. N. Ghosh, A. Bauder, and Hs. H. Giinthard, Chem. Phys., 53,
Experimental Section Sample Preparation. Methyl nitrite was synthesized by dropwise addition of cold 6 N sulfuric acid to a stirred aqueous solution of reagent-grade sodium nitrite and reagent-grade methanol which was maintained at -5 f 1 O C 9 The product was collected in a trap maintained at ---78 "C and was thoroughly degassed and subsequently purified by repeated vacuum distillations. Purified methyl
39-50 (1980).
(2) P. Felder, T.-K. Ha, A. M. Dwivedi, and Hs. H. Gunthard, Spectrochem. Acta. Part A. 37. 337-45 (1981). (3) P. T. Inglefield, E. Krakower,'L. W. Reeves, and R. Stewart, Mol. Phys., 15, 65-86 (1968). (4) F. R. Cordell, J. E. Boggs, and A. Skancke, J . Mol. Struct., 64, 57-66 (1980). . . . _. ~ ..,
(5) P. Felder and Hs. H. Gunthard, Chem. Phys. Lett., 66, 283-6 (1979). (6) P. N. Ghosh and Hs. H. Giinthard, Spectrochim. Acta, Part A , 37, 347433 (1981). (7) F. L. Rook and M. E. Jacox, J . Mol. Spectrosc., 93, 101-16 (1982).
(8) P. H. Turner, M. J. Corkill, and A. P. Cox, J . Phys. Chem.. 83, 1473-82 (1979)
(9) W. A. Noyes in 'Organic Syntheses", Collect. Vol. 11, E. C. Honning, Ed., Wiley, New York, 1943, p 108.
0022-3654/83/2087-1622$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistv, Vol. 87, No. 9, 1983
NMR Studies of Methyl Nitrite
nitrite samples were stored at -35 "C under vacuum and showed no evidence of decomposition over a several-month period as demonstrated by infrared and 'H NMR spectroscopy. Gas-phase NMR spectra of methyl nitrite were obtained by using a sealed 12 mm diameter sample tube containing 60 torr of methyl nitrite and approximately 1 torr of Me4Si. Additional low-temperature gas-phase data were obtained by using a sample containing 27 torr of methyl nitrite and 40 torr of methane. Liquid samples included neat methyl nitrite and approximately 1% (by volume) solutions of methyl nitrite in acetone-d6 (Aldrich Gold Label), carbon disulfide (Mallinckrodt analytical reagent), and n-pentane (Aldrich). All liquid samples were thoroughly degassed and sealed under vacuum. Spectroscopic Measurements. All NMR measurements were obtained by using a 4.8-T wide-bore Nicolet spectrometer, with proton observation at 200 MHz. Gas-phase NMR spectra of methyl nitrite were obtained at 3 K temperature intervals between 193 and 223 K. Accessible temperatures were limited by sample volatility and chemical exchange at the low- and high-temperature limits, respectively. T I for syn- and anti-methyl nitrite at 60 torr and 223 K are ca. 0.32 and 0.29 s, respectively. Intensity data were obtained by using a 5-ps rf pulse (45" flip angle) and a 6-s delay between pulses to ensure that the intensity ratios were not perturbed by the acquisition process. Typically, 72 transients were collected, summed, and Fourier transformed to yield frequency domain spectra with signal-to-noise ratios of at least 500/1. Line widths were ca. 4 Hz over the temperature range 193-223 K. Additional spectra were obtained in the fast-exchange limit above 325 K. Measurements were performed on liquid samples with a 0.5-ps rf pulse (6" flip angle) and a 30-s pulse delay time which is more than five Tl's for all samples and temperatures studied. Measurements were obtained at 3 K temperature intervals between 223 and 184 K. Typically, eight transients were collected, summed, and Fourier transformed to yield frequency domain spectra with signal-tonoise ratios of 500/1. Typical line widths in the liquid samples were ca. 2 Hz. For these concentrated samples it was necessary to employ an rf attenuator to decrease the intensity of the free induction decay signals by a factor of 9 for the 1% solutions of methyl nitrite in carbon disulfide and acetone-d, to a factor of 27 for the neat and 1% in n-pentane solutions in order to eliminate power distortions resulting from amplifier overload. Frequently, conformer population ratios are obtained from the temperature dependence of chemical shifts in rapidly exchanging ~ y s t e m s .Since ~ the magnitude of the temperature-dependent rovibrational contribution to the chemical shielding is not known for gaseous methyl nitrite, it was not possible to obtain reliable estimates of populations from chemical shifts in the fast-exchange region. Each intensity ratio reported in Table I is the average of five integrations. Enthalpy differences between conformers were obtained from the temperature dependence of the spectral intensity ratios by using an unweighted least-squares regression analysis. Uncertainties in reported results are two standard deviations. Results A typical NMR spectrum of methyl nitrite gas at 60 torr and 205 K is shown in Figure 1. The signal/noise ratio, ca.500/1, and flat base line yield integrated intensity ratios with 1-2% associated uncertainties. The downfield resonance at 4.578 ppm relative to gaseous Me,Si (0.00 ppm) and the upfield resonance at 3.504ppm can be unambig-
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TABLE I : Anti/Syn Conformer Intensity Ratios for Methyl Nitrite 1 % solution
T,K 184 187 190 193 196 199 202 205 208 211 214 217 219 223 226
in acetoned,
neat liquid
gas
0.2555" 0.2705 0.2790 0.2866 0.2956 0.3030 0.3126 0.3218 0.3339 0.3445 0.3542 0.3636 0.3650 0.3775 0.3760
0.2477' 0.2553' 0.2636' 0.2718" 0.2788 0.2871 0.3004 0.3104 0.3208 0.3292 0.3422
0.2450 0.2580 0.2637 0.2749 0.2856 0.2926 0.3101 0.3223 0.3254 0.3330 0.3423 0.3483
in CS,
0.2750 0.2999 0.3056 0.3210 0.3369 0.3358 0.3456 0.3518 0.3664 0.3740
in n-pentane
0.2670 0.2729 0.2780 0.2907 0.2986 0.3081 0.3273 0.3377 0.3361 0.3502 0.3668
a Uncertainties are k0.0005 for all gas- and liquid-phase measurements. Data for a sample containing 27 torr o f methyl nitrite and 40 torr of methane. Uncertainties are k0.003.
'
6
5
tl
3
2
PPM
Figure 1. Gas-phase 'ti NMR spectrum of a 60-torr sample of methyl nitrite obtained at 205 K. Resonances at 4.5780 and 3.5035 ppm relative to gaseous Me& arise from the anti and syn conformers, respectively. The spectrum is the Fourier transform of the sum of 72 transients.
uously assigned to the anti and syn conformers, respectively, based on results of microwave and infrared studies which demonstrate that the syn conformer is the most stable. Temperature-dependent spectral intensity data for methyl nitrite gas and neat methyl nitrite liquid appear in Figure 2. For gaseous methyl nitrite, the syn/anti intensity ratio ranges from 4.037 at 193 K to 2.922 at 223 K, corresponding to a 28% change in this ratio over a 30 K interval. For liquid methyl nitrite, the syn/anti intensity ratio ranges from 3.489 at 193 K to 2.649 at 223 K corresponding to a 24% change in this ratio over the same 30 K temperature interval. Additional data for methyl nitrite solutions appear in Table I. Associated thermodynamic parameters are listed in Table 11. The enthalpy difference between the two methyl nitrite conformers, obtained from least-squares analysis of the temperature-dependent intensity data, is 998 (50) cal mol-' for gaseous methyl nitrite and 818 (31) cal mol-l for liquid methyl nitrite. Within experimental uncertainty limits all the solutions studied have anti-syn enthalpy differences similar to the neat liquid. For all samples studied, the conformer free energy
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The Journal of Physical Chemisrry, Vol. 87,No. 9, 1983
Chauvel and True
TABLE 11: Relative Intensities and Relative Thermodynamic Parameters of r y n - and anfi-Methyl Nitritea statistical mechanical in n-pentane calcn
1%solution
60 torr of gas
AH^^^-^^^, cal mo1-I
998 ( 5 1 ) A G ~ ~ cal ~ mol-' - ~ ~ ~ : 520 ( 3 ) A'Smti-syn, cal mol-' K-' 2.33 ( 2 5 ) zanti/lsyn (205 K ) 0.2788 ( 5 ) z ( an t i ) / z (syn) 3.24 ( 4 0 ) solvent internal press., a t m 0 1 solvent dielectric constant a
neat liquid
in acetone-d,
in CS,
818 ( 3 ) 443 ( 2 ) 1.83 ( 1 5 ) 0.3218 ( 2 ) 2.41 ( 1 9 )
866 ( 5 1 ) 460 ( 2 ) 1.98 ( 2 5 ) 0.3223 ( 2 ) 2.67 ( 3 5 ) 3282 20.7
803 ( 5 0 ) 443 ( 2 ) 1.76 ( 2 5 ) 0.3369 ( 2 ) 2.40 ( 3 6 ) 3715 2.6
832 ( 6 9 ) 443 ( 2 ) 1.90 ( 2 5 ) 0.337 ( 2 ) 2.56 ( 4 4 )
1045 520 2.56 0.309 3.59
2.1
At 205 K. SY N
700
600
2
2 2
Flgure 2. Temperaturedependence of the population ratios of the syn and anti conformers for gas-phase (A)and liquid (B) samples. The estimated uncertainties of f0.002 in the logarithms of the intensity ratios are invisible on this scale.
differences, AGantiayn, are consistently much smaller than the enthalpy differences. AGanti* for gaseous methyl nitrite is 520 (3) cal mol-' and values for the liquids are ca. 450 cal mol-' at 205 K. Accordingly, in all cases the entropy of the anti conformer is significantly greater than that of the syn form. Since for both conformers ASanti* is relatively large, AGmtiayn is very temperature dependkt and is calculated to be ca. 303 cal mol-' for gaseous methyl nitrite and ca. 272 cal mol-' for liquid-phase samples at 298 K by using data appearing in Table 11. These results agree with NMR frequency data for methyl nitrite obtained under high-temperature, fast-exchange conditions for both gas- and liquid-phase samples. Although the gas-to-liquid enthalpy and free energy differences obtained are small, they are significant. Nine data points for each sample were obtained with signal/ noise ratios of ca. 500/ 1, and multiple integrations demonstrate a less than 2% uncertainty in the resulting intensity ratios.
AEanti-syn
Discussion
The experimentally determined thermodynamic parameters for the methyl nitrite conformational equilibrium can be compared with a statistical mechanical calculation, results from other experimental and theoretical studies, and probable solvent perturbation effects. For comparison with our experimental gas-phase results, the anti/syn methyl nitrite partition function ratio and associated thermodynamic functions were calculated by standard statistical methods. Moments of inertia determined from a recent high-resolution microwave study were used.a For anti-methyl nitrite, the product of the three principal moments is 1.1554 X lo5 amu3 A6, and for the
060 T ( NOCH,)
Flgure 3. Methyl top internal rotation potential functions of syn- and anti-methyl nitrite showing energy levels (cm-') and degeneracies (9). For syn-methyl nitrite V , = 734 (2) cm-' and B o (the internal rotation constant) = 5.6817 cm-'; for anti-methyl nitrite V , = 10.1 cm-' and 6, = 6.9834 cm-'.
less prolate syn conformer it is 1.5203 X lo5 amu3 k6 yielding a rotational partition function ratio, z,(anti)/z,(syn), of 0.87176. For each conformer of methyl nitrite, 13 of the total 15 vibrational frequencies have been obtained directly from infrared absorption measurement^.^ Vibrational frequencies associated with the N-0 and 0-C torsional motions have also been determined for both conformers. Frequencies for the N-0 torsional vibration have been obtained from hot band progressions and combination b a n d ~ . ~ Hot f j band progressions arising from the N-0 torsion for syn-methyl nitrite have been identified for a number of fundamental modes. Band contour simulations for three vibrational transitions of syn-methyl nitrite are consistent with an NO torsional frequency, UNO,of 269 (16) cm-'. For anti-methyl nitrite, U N O obtained from combination band analysis is estimated to be 180 cm-'. The contributions of the N-0 torsional vibrations to the total vibrational partition functions were calculated neglecting anharmonicity contributions which are estimated to be small for the low excited levels populated at the low temperatures where we obtained experimental data. The V3 methyl top barriers in syn- and anti-methyl nitrite have been calculated from observed splittings in their associated microwave spectra.'B8 These barriers are remarkably different 734 (2) cm-' for syn-methyl nitrite and only 10.1 cm-' for anti-methyl nitrite. Associated energy levels were calculatedloby using an internal rotational constant, Bo,
-
(10) J. D.Lewis, J. B. Malloy, Jr., T. H. Chao, and J. Laane, J . Mol. Struct., 12, 427-49 (1972).
NMR Studies of Methyl Nitrite
The Journal of Physical Chemlstry, Vol. 87, No. 9, 1983
of 5.6817 cm-' for the syn conformer and 6.9834 cm-l for the anti conformer which were calculated from their respective r8 structures." The resulting levels are shown in Figure 3. The total vibrational partition function ratio, z,(anti)/z,(syn), was calculated over our experimental temperature range by using the vibrational frequencies and torsional levels discussed above. The large positive value (3.76 at 200 K) of the ratio z,(anti)/z,(syn) is due primarily to the methyl top internal rotation potential functions which are shown in Figure 3. The lower barrier in antimethyl nitrite has a much higher associated vibrational state density than that associated with the methyl top barrier for syn-methyl nitrite. Below 200 cm-l, anti-methyl nitrite has 13 methyl top vibrational states compared to 6 for the syn conformer. Associated thermodynamic parameters, calculated from the above vibrational and rotational data at 205 K, appear in Table 11. Excellent agreement between our gas-phase NMR data and the results of the statistical calculation is obtained by using an assumed anti-syn vibrational ground-state energy difference, Ace, of 350 cm-'. Agreement between our results and the microwave' and ab initio2r4results is good. However, our value of AH is significantly higher than that obtained in the beam deposition experimenb2 Similar beam deposition experiments have, in some cases, yielded lower AH values than those obtained by other t e c h n i q ~ e s . ' ~ -A~ ~previous neat liquid NMR study which reported a conformer relative enthalpy difference close to zero utilized high-temperature frequency data to obtain population^.^ However, two temperaturedependent effects determine the frequencies of the methyl nitrite proton resonance under fast-exchange conditions. Relative populations shift with temperature, and also the magnitude of the chemical shielding in polar liquids is temperature dependent. We observe a linear upfield shift of -0.2 Hz OC-' for both conformers over the temperature range 184-226 K. This effect, which balances the downfield shift resulting from an increasing anti population with increasing temperature, was neglected in the earlier NMR analysis, resulting in an erroneously small conformer enthalpy difference. The thermodynamic parameters describing the methyl nitrite syn + anti equilibrium are phase dependent. For methyl nitrite, the ratio of the conformer cavity distribution functions, ya*/ys*, for each solution can be calculated from experimentally determined equilibrium constants:
(Xa/Xs) = (Xa/Xs)o(Ya*/Ys*)
(1)
where X denotes a mole fraction, subscripts s and a denote the syn and anti conformers, and the superscripted O indicates the gas-phase r e ~ u l t . ' ~ JAt ~ 205 K, ya*/ys* is 1.154, 1.156,1.208,and 1.211 for the neat liquid and 1% solutions in acetone-d,, CSz,and n-pentane, respectively. For methyl nitrite, steric and dielectric factors can contribute to the (11) S. R. Polo, J. Chem. Phys., 24, 1133-5 (1956). The computer program CART,written by H. M. Pickett, calculates Gifor trial structures. (12) P. Felder and Hs. H. Gunthard, Chem. Phys. Lett., 66, 283-6 (1979). (13) P. Huber-Wdchli and Hs. H. Gunthard, Chem. Phys. Lett., 30, 347-51 (1975). (14) C . E. Blom, R. P.Muller, and Hs. H. Giinthard, Chem. Phys. Lett., 73, 483-6 (1980). (15) P. Felder and Hs. H. Gunthard, Spectrochim. Acta, Part A , 36, 223-4 (1980). (16) P. Huber-Walchli and Hs. H. Gunthard, Spectrochim. Acta, Part A , 37, 285-304 (1981). (17) P. N. Ghosh, R. Gunde, and Hs. H. Gunthard, Spectrochim. Acta, Part A, 38, 8-18 (1982). (18) D. Chandler and L. R. Pratt, J.Chem. Phys., 65,2925-40 (1976). (19) L. R. Pratt and D. Chandler, J. Chem. Phys., 68,4202-12 (1978).
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relative magnitudes of the conformer cavity distribution functions. The phase dependence of conformer equilibria has been investigated theoretically for nonpolar systems, where cavity distribution functions can be calculated from hard-sphere models. The phase dependence of the trans gauche equilibrium of butane has been calculated from hard-sphere cavity distribution functions using statistical mechanical methods and an approximate two-cavity model.1es20 The cavity distribution function of gauchebutane is approximately twice as large as that of the trans conformer in condensed phases since the effective volume that it occupies is smaller. For butane, the conformer molar volume difference, AV(gauche-trans) is --4.7 cm3 mol-' in CCl, and --2 cm3mol-' for the neat liquid, since the methyl-methyl C-C distance is smaller for the gauche form.lg syn- and anti-methyl nitrite have different molar volumes and dipole moments. Therefore, both steric liquid packing and dielectric effectsz1can perturb the conformer equilibrium from its gas-phase value. In order to ascertain the steric effects on this system AV(anti-syn) was calculated by assuming van der Waals radii of 1.50, 1.40, and 2.00 A for N, 0, and CH, groups, respectively, and using the r8geometries of syn- and anti-methyl nitrite. The resulting intrinsic value of AV(anti-syn) is 2.44 cm3 mol-'. The anti conformer is larger because the methyl and oxygen van der Waals spheres do not overlap. On the basis of analogy with the butane calculations, one would expect that ya*/y8*for methyl nitrite would be ca. 0.5 if only steric liquid packing forces were important in this system. However, anti-methyl nitrite has a dipole moment, wt(anti) = 2.37 D, slightly larger than that of syn-methyl nitrite, &yn) = 2.050. Since the anti conformer is slightly more polar, dielectric effects will tend to increase the value of the cavity distribution function of anti-methyl nitrite relative to the syn form. The observed modest increase in the anti/syn conformer ratio for methyl nitrite in condensed phases may be the result of balance between steric and dielectric effects. Although the factors which cause increased stabilization of anti-methyl nitrite relative to the syn conformer in condensed phases are complex, a possible explanation for the lower anti-syn free energy difference can be proposed. The closer similarity of the syn and anti partition functions in condensed phases may be due to increased similarity in their V3methyl top potential functions. No direct experimental evidence pertinent to the relative V, methyl top barrier heights in condensed-phase methyl nitrite samples is available. However, a comparison of the 13 highest vibrational frequencies for gaseous7and liquid22 methyl nitrite indicates little phase dependence and cannot account for this difference. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the National Science Foundation through grant CHE-8210844, the University of California, Davis (UCD) Committee on Research, and the UCD NMR facility for additional support. Registry No. Methyl nitrite, 624-91-9. (20) C. S.Hsu,L. R. Pratt, and D. Chandler, J. Chem. Phys., 68, 4213-7 (1978). (21) See, for example, R. J. Abraham and E. Bretschneider in "Internal Rotation in Molecules", W. J. Orville-Thomas, Ed., Wiley, New York, 1974, Chapter 13, pp 480-584. (22) P. Klaboe, D. Jones, and E. R. Lippincott, Spectrochim. Acta, Part A , 23, 2957-71 (1967).
