7480
J. Phys. Chem. 1993,97, 1480-1483
Conformational Studies of Alkyl Nitrites Using Low-Temperature 13C, 170, and 14N NMR Spectroscopy. 170NMR of N-Substituted Formamidesl Diwakar Pawar, Hugh L. Mark, Hasan Hosseini, Yolanda Harris, and Eric A. Noe' Department of Chemistry, Jackson State University, Jackson, Mississippi 3921 7-051 0 Received: April 22, I993
Low-temperature 13C, 170, and 14N N M R studies of alkyl nitrites, RONO, show that N M R spectra of these nuclei can be used in assigning conformations. Signals for the E and 2 conformations are well-resolved for the 1 7 0 spectra in both the N-== and N - O regions, with the E isomers absorbing at higher frequency than the Z isomers. Compounds which were studied included R = methyl (I), ethyl (2), propyl (3), butyl (4), isopropyl (5), sec-butyl (a), and tert-butyl (7). The chemical shifts are suggested to be related to the longerwavelength absorption of the E conformations in the UV spectra. The signals for E and Z isomers can also be seen in the low-temperature 14N spectra, although there is some overlap of peaks, except for methyl nitrite. The E isomers also absorb at higher frequency in the 14N and 13C spectra. N-substituted formamides were studied by I7ON M R , and the Zconformations of N-tert-butyl formamide and formanilide were found to absorb at higher frequency for these compounds.
Introduction Tarte showed in 1952 that the gas-phase IR and UV spectra of alkyl nitrites could be interpreted in terms of a mixture of E and Z conformations.2 Peak assignmentsfor methyl nitrite were R-
0
\ N=
E
e 0
7
o
\
N-
0 2
made with the assumption that the more stable conformation of this compound was 'chelated", with a methyl proton hydrogen bonded to oxygen. Changes in the two N-0 band intensities then indicated that the populations of the E isomers increase in the order R = CHJ < primary < secondary < tertiary. Assignments of major and minor conformations in the first two dynamic NMR studies*.9of alkyl nitrites were opposite those of Tarte.2 Piette et a1.8 also assumed that an a-proton was hydrogen-bondedin the 2 conformation and assigned the higher frequency a-proton peaks to the Z isomers on this basis. Phillips et aP observed that one of the conformations gave a-proton chemical shifts close to the values for alcohols and ethers and concluded that these peaks, at lower frequency, must be due to the E isomers, as the chemical shifts in the Z isomers would be affected by the N I O group. The high-frequency/low-frequency area ratio of 0.13 reported by Phillips9 for the a-protons of isopropyl nitrite is incorrect, and later workers reported ratios of 16I0and 15." Gray and Reeves suggested12that the discrepancy between the gas-phase (IR2 and electron diffraction3)and solution (NMR)"lOJ2 results could be due to stabilization of the more polar E conformation in solution. Another explanation was proposed in 1964 by Brown and Hollis,I1who provided convincing evidence that the previous lowtemperature NMR peakassignments8-loJ2were incorrect. NMR spectra of 1,2, and 5 were obtained" for the pure liquids and for solutions in CDCl3, CD30D, and CC14. No changes were observed in either the E/Z ratios or the chemical shifts, and they concluded that internal hydrogen bonding is not significant in determining either the relative stabilities of E and Z conformations or the chemical shifts. Other support" for the revised assignment came from a consideration of steric effects in methylalkylnitrosamines and alkyl nitrites. Conformations with the alkyl groups trans to the
nitroso group should be preferred in the nitrosamines because of steric effects, and conformational assignments made with this assumption were consistent with the expected anisotropic diamagnetic shielding of the nitroso group, which would cause a-protons cis to N = O to be shifted to lower frequency. A consistent assignment of a-proton peaks could also be made in alkyl nitrites by assuming that the Z conformations absorb to lower frequency of E and that the compounds with larger alkyl groups should have larger populations of E conformations. In this way, E / Z ratios of 0.26, 2.4, 2.4, and 15 were determined for compounds 1,2,3, and 5. Similar results for these compounds were obtainedI3J4in more recent, detailed studies. Twenty years after Brown and Hollis published their work, Lazaar and Bauer15 reported that the major conformations of three primary nitrites (ethyl, propyl, and butyl) are 2. In part, this conclusion was based on a microwave study16 of ethyl nitrite, in which the following four conformations were considered. Energies in cm-l relative to the most stable conformation are given in parentheses for the three isomers that were observed. H
H
\ C IH HsC/
\0-N
0
/o
0-N
-
trans 2 (0)
-
gauche 2 (238)
-
gauche E (81)
trans E (not found)
-
From these data, the 2 isomer is lower in energy than the E. Lazaar and Bauer15 observed that the more intense a-proton absorption in the slow-exchange NMR spectra of ethyl nitrite was at higher frequency in both the gas phase and solution, which provided evidence that the major conformation was the same in the two phases, and they assigned the larger peaks to thezisomer. This conclusion has been criticizedI4on the basis of the expected
0022-3654/93/2091-1480~04.00/0 0 1993 American Chemical Society
Conformational Studies of Alkyl Nitrites
The Journal of Physical Chemistry, Vol. 97,No. 29, 1993 7481
TABLE I: Experimental Conditions for NMR Spectra parameter 1 7 0N M R 14N N M R 13C N M R spectrometer 40.76 21.71 75.57 frequency (MHz)
pulse width (ps)/ flip angle 30/90° delay time (s) 0.0005
63/90° 0.5or 1
22/90° 5 (nitrites) 13-23 (amides) f7000 f 100 k3800 f 100
sweep width (Hz)
f20 000
line broadening
20 (nitrites) 3 30 (formamides) 3 16 K 32 K
3 (amides) 32 K
10 000-30 000
290 - 591
(Hz)
no. of data points no. of acquisitions
(nitrites)
500 f 100
f19 230 (amides)
5 (nitrites)
shielding effect of the nitroso group upon the a-protons.11J7 True and co-workersl3J4 have determined AHo and ASofor conformational equilibration of several nitrites and showed that the enthalpy change for the E Z interconversion of ethyl nitrite favors the Z isomer, but the - T A P term is larger and favors the E isomer, causing it to predominate. The energies determinedl6 by microwave spectroscopy for ethyl nitrite do not include an entropy term and therefore are not directly related to the populations. We report low-temperature I3C, I4N,and I7Ostudies which support the conclusions of Brown and Hollis.11 The E-Z chemical shift differences for low-temperatureI7Oand I4Nspectra are shown to be large enough to be useful in studying the conformational equilibra of alkyl nitrites, although the lines are broad. By contrast, the E-Z chemical shift differences for I7O spectra of N-substituted formamides are generally much smaller, and a possible explanation for the difference in the two systems is described. Experimental Section Alkyl Nitrites. Propyl, isopropyl, sec-butyl, and tert-butyl nitrites were purchased from Pfaltz and Bauer, and tert-butyl nitrite was also obtained from Aldrich Chemical Company. The purity of these compounds was determined using proton NMR, and they were purified, if necessary, by distillation. Methyl nitrite was synthesized by an Organic Syntheses procedure,'* and ethyl nitrite was prepared by a procedure similar to one described for butyl nitrite.Ig NMR Studies. Spectra were taken on a General Electric GN300 wide-bore, FT spectrometer. The experimental conditions used to study the different nuclei are summarized in Table I. The temperature was measured after each low-temperature spectrum, as described previously.20 Neat samples were used for 14N and 170 spectra of the alkyl nitrites, and 20% solutions in CD2C12 were used for 13Cand 1Hstudies of these compounds. The samples used for the N-substituted formamides were 30% solutions in CHC12F. Results and Discussion
IC Spectra. Slow-exchange a-carbon chemical shifts have been reported15 for 1 and 2, and our results for these compounds are summarized in Table I1 along with our data for 5. The chemical shifts in this table are assigned so that the relative populations are in agreement with those of Brown and Hollis.11 With these assignments, the E conformations consistently absorb at higher frequency than the Z isomers. Lazaar and Bauer15 used the opposite assignment for ethyl nitrite, which would require switching of the relative positions of E and Z 13C signals on going from methyl to ethyl, and reversal of positions again on going from ethyl to isopropyl. Similarly, the a-proton chemical shifts would be switched twice on going from methyl to ethyl to isopropyl.
TABLE Ik Internal TMS compd methyl nitrite ethyl nitrite -CHo
-CH2 isopropyl nitrite -CHp -CH
Chemical Shifts of Alkyl Nitrites, Relative to chemical shifts (ppm) E Z
temp (OC)
58.6
52.5
-49.4
16.1 67.5
12.0 61.2
-48.2
22.7 74.9
20.1 67.0
-39.8 -39.8
TABLE IIk 1 7 0 Chemical Shifts of Alkyl Nitrites at -31 "C in ppm, Relative to External Water at Room Temperature N=O
compd methyl nitrite ethyl nitrite n-propyl nitrite isopropyl nitrite n-butyl nitrite sec-butyl nitrite tert-butylnitrite
E 832.2 832.8 829.7 832.3 830.2 828.8 843.5
N-O
Z 791.5 796.4 794.0 795.3 794.6 791.8
E 451.9 482.4 478.5 503.1 478.6 501.0 512.3
Z 414.5 443.1 438.6 465.4 438.6 462.3
TABLE I V 14N Chemical Shifts of Alkyl Nitrites, Relative to External 30 Aqueous Sodium Nitrate chemical shifts (ppm) compd E Z temp (OC) methyl nitrite 205.2 177.9 -41.0 ethyl nitrite 205.3 179.8 -50.0 n-propyl nitrite 203.4 178.9 -40.2 isopropyl nitrite 209.8 -40.0 tert-butyl nitrite 209.5 -45.2 170Spectra. Room-temperature I7Ospectra have been reported for several alkyl nitrites,*1-23 and the higher frequency signals were assigned to the N 4 oxygens. I7Ochemical shifts obtained a t low temperatures in this work are summarized in Table 111, and representative spectra are shown in Figure 1. The signals for E and Z conformations are well-resolved a t -31 OC,with the E isomers absorbing at higher frequency in both the N-0 and N-0 regions. Only the E isomer of 7 is observed, due to the low population of the Z conformation, which has not been reported.14.2425 14N Spectra. Room-temperature I4N N M R spectra of alkyl nitrites have also been rep~rted:~-~~ and from the chemical shifts of 1,2,4, and 7, the authors concluded that the E isomers absorb at higher frequency than the 2 isomers. However, accurate chemical shifts for individual conformations could not be obtained from these averaged spectra. The I4N spectra of methyl nitrite a t three temperatures are shown in Figure 2, and low-temperature spectra of several compounds are compared in Figure 3. Some overlap of peaks occurs, except for methyl nitrite. The peaks of Figure 3 may still be broadened somewhat by exchange, but better resolution of signalsat lower temperatures was not feasiblebecause of rapid relaxation. Chemical shifts are summarized in Table IV. The E / Z ratios are similar to those of ref 11 if the higher frequency signals are assigned to the E isomers. N-Substituted Formamides. A single 1 7 0 peak was reported for N-tert-butylformamide (8) (6 323)Mand N-methylformamide (6 304)" as neat liquids. The chemical shifts for 8 were resolved for E and Z isomers in aqueous solution (6 267 and 286, respectively).30 In our work, 1 7 0 N M R spectra were taken a t slightly elevated temperatures in CHClZF, and chemical shifts are given in Table V. The table also includes the percentage of the E conformation for each sample. These percentages were obtained from integration of the 13C spectra, with the highfrequency carbonyl 13C peak assigned in each case to the E conformation.32 The more intense I7O signal of N-fert-butylformamide must come from the Z isomer and absorbed 16.3ppm
Pawar et al.
7482 The Journal of Physical Chemistry, Vol. 97,No. 29, I993
TABLE V:
Chemical Shifts of 30% N-Substituted Formamides in CHCIZF,Rdrtive to External Water at Room Temperature line widths (Hz) chemical shifts (ppm) % of E isomers "pd E Z E Z temp ("C) from 13C spectra 207 205 45.4 45.6 N-rerr-butylformamide 306.6 322.9 45.1 44.1 formanilide 331.5 338.3 204 44.3 23.5 316.3 N-cyclopropylformamide 170
l
R = CH3CHz
l
I
= (CH,J,CH
*
9608bO7bSb05bO PPM Figure 1. Low-temperature I7O NMR spectra of alkyl nitrites.
+18.OoC
)\
A -41 .o
220
200
160
180
PPM
Figure 2. I4N NMR spectra of methyl nitrite.
dA+ R = CH,CH,CH,
(6.8 ppm) is smaller than for 8. The chemical shifts for 9 are at higher frequency than for either 8 or 10, as a consequence of a slightly reduced interaction of the nitrogen lone pair with the carbonyl group in 9, resulting from phenyl substitution. The populations of the E and Z isomers of 9 are nearly equal, and the higher frequency signal is assigned to the Z isomer by analogy to 8.34 The relative positions of I7Osignals for E and Zconformations of the formamides are opposite those of thealkyl nitrites, indicating that the origin of the chemical shift differences is different for the two classes of compounds. The E conformations of alkyl nitrites are known2 to absorb at longer wavelength in the UV,and the relative chemical shifts for the two conformations can be expected because of the lower average excitation energy for the E isomers. A similar explanation may also account for the I4N chemical shifts. As noted above, the proton chemical shifts can be understood in terms of the expected anisotropic diamagnetic shielding of the nitroso group. For all of the nuclei studied-aprotons, a-carbons, 14N, and both oxygens-the E isomers then absorb at higher frequency. The assignment by Lamar and Bauerls of the major conformation of primary alkyl nitrites to the 2 isomer would require that the positions of peaks for all of thesenuclei beswitched ongoingfrommethyl toethyl and reversed again on going from ethyl to isopropyl, which is unlikely.
Conclusions The 1 7 0 results for alkyl nitrites show that the spectra are generally useful in assigningconformations for these compounds, with the E isomers absorbing at higher frequency. The relative chemicalshiftsare suggestedtobe related to the longer wavelength absorbance of the E isomers in the UV spectra. Low-temperature 14N spectra also show signals for E and Zisomers, but the chemical shift differences are small enough that some overlap of peaks occurs, except for methyl nitrite. The two conformations of N-substituted formamides should absorb at similar wavelengths in the UV spectra, and the I7O peaks were resolved only if the substituent on nitrogen was large (e.g., tert-butyl or phenyl), which causes a shift to higher frequency for the 2 isomer as a consequence of steric interactions. Acknowledgment. We thank the National Institutes of Health (Grant No. S06GM08047) for support of this work and the National Science Foundation (Grant No. RII-8405345) for part of the funds used to purchase the wide-bore GN300 NMR spectrometer. References and Notes
I
210
"
200
'
"
180
'
"
1 6 0 PPM
Figure 3. Low-temperature 14NNMR spectra of alkyl nitrites. at higher frequency than the E isomer. Steric interactions of alkyl groups with carbonyl oxygens are knownmJ3 to shift the 1 7 0 signals to higher frequency, and this effect could account for the relative shifts of the E and 2 isomers of 8. N-Cyclopropylformamide (10) shows only a single 1 7 0 peak with a line width of 204 Hz, although 23.5% of the E isomer is present. The cyclopropyl group is apparently too small to significantly affect the chemical shift of the 2 isomer. The phenyl group of formanilide (9) is intermediate in size between the tert-butyl and cyclopropyl groups, and two overlapping 1 ' 0 signals can be observed for this compound, although thechemical shift difference
(1) Parts of this work have been described at the following meetings: (a) Hosseini, H.; Smith, V.;Noc, E.A. 192nd National ACS Meeting. Anaheim, California, September 12, 1986; Abstract No. 312. (b) Mark, H.; Hosscini, H.; Noc, E. A. 195th National ACS Meeting, Toronto, Canada, June 9,1988; Abstract No. 402. (c) Noe, E. A.; Mark, H.; Hosseini, H. 16th Annual NIHMBRS Symposium, Los Angeles, California,October 15,1988. (d) Hams, Y.;Pawar, D.;Mark, H.; Gallo, A.; N a , E.A. NIGMS Minority Programs Symposium, Washington, D.C., November 5, 1991. (2) Tarte, P. J . Chem. Phys. 1952, 20, 1570. The first evidence for rotationalisomerism in alkyl nitrites was obtained by Rogowski from electron diffraction data.3 For discussions of the early spearoscopic studies of the conformations of alkyl nitrites, see refs 4-7, and referen- cited therein. (3) Rogowski, F.Ber. 1942, B75,244. (4) Haszeldine, R. N.; Jaader, J. J. Chem. Soc. 1954, 691. (5) Klaboe, P.; Jones, D.;Lippincott, E. R. Spectrochlm. Acta 1967, 23A, 2957. ( 6 ) Gray, P.; Pearson, M. J. Trans. Faraday Soc. 1963,59, 347. (7) Ogilvie, J. F. J. Chem. Soc., Chem. Commun. 1973,450. ( 8 ) Piette, L. H.; Ray, J. D.;Ogg, R. A. J . Chcm. Phys. 1957,26,1341.
Conformational Studies of Alkyl Nitrites (9) Phillips, W. D.;Looney, C. E.; Spaeth, C. P. J.Molecular Spectrosc. 1957,I , 35. (10) Piette, L. H.; Anderson, W. A. J. Chem. Phys. 1959,30, 899. (11) Brown, H. W.; Hollis, D. P. J. Mol. Spectrosc. 1964,13, 305. (12) Gray, P.;Reeves, L. W. J. Chem. Phys. 1960,32,1878. (13) Chauvel, J. P.,Jr.; True, N. S . J. Phys. Chem. 1983,87, 1622. True, N. S.;Ott, (14) Conboy, C. B.; Chauvel, J. P.,Jr.; Moreno, P. 0.; C. M. J. Phys. Chem. 1986.90.4353. (15) L a i a r , K. I.; Bauer, S.H. J. Phys. Chem. 1984,88,3052. (16) Turner, P.H. J. Chem. SOC.,Faraday Trans. 2 1979,75, 317. (17) Harris, P. K.; Spragg, R. A. J. Mol. Spectrosc. 1967,23, 158. (18). Hartung, W. H.; Crossley, F. In Organic Syntheses; Blatt, A. H., Ed.; Wiley: New York, 1943;Collect. Vol. 11, p 363. (19) Noyes, W. A. In Organic Syntheses; Blatt, A. H., Ed.; Wiley: New York, 1943;Collect. Vol. 11, p 108. (20) Mark, H.; Baker, T.; Noe, E. A. J. Am. Chem.Soc. 1989,111,6551. (21) Figgis, B. N.; Kidd, R. G.; Nyholm, R. S.Proc. Roy. Soc. ( A ) 1962, %O a ",,
AAQ _VI.
(22) Andersson, L.-0.; Mason, J. J. Chem.Soc.,Dalton Trans. 1974,202. (23) Sugawara, T.;Kawada, Y.; Kat&, M.; Iwamura, H.Bull. Chem. Soc. Jpn. 1979,52,3391. (24) True, N. S.;Bohn, R. K. J. Phys. Chem. 1982,86,2327. (25) Witanowski, M.; Sitkowski, J.; Biernat, S.;Kamienski, B.; Hamdi, B. T.; Webb, G. A. Magn. Reson. Chem. 1985,23,748.
The Journal of Physical Chemistry, Vol. 97, No. 29, I993 7483 (26) Andersson, L.-0.;Mason, J.; van Bronswijk, W. J. Chem. Soc. ( A ) 1970.296. (27) Mason, J. J. Chem. Soc., Faraday Trans. II 1976,2064. (28) Mason, J.; van Bronswijk, W.; Vinter, J. G. J. Chem. Soc., Perkin Trans 2 1977,469. (29) Sicinska, W.; Stefaniak, L.; Witanowski, M.; Webb, G. A. J. Mol. Srruct. 1987. 160. 179. (30) Valentine, B.; Steinschneider, A.; Dhawan, D.; Burgar, M. I.; St. Amour, T.; Fiat, D. Int. J. Pept. Protein Res. 1985,25, 56. (31) Burgar, M. I.; St. Amour, T. E.; Fiat, D. J. Phys. Chem. 1981,85, 502. (32) (a) Dorie, J.; Gouesnard, J. P.; Mechin, B.; Naulet, N.; Martin, G. J. Org.Magn. Reson. 1980,13,126. (b) Llinares, J.; Faure, R.; Vincent, E. J.; Elguero, J. Spectrosc. Lett. 1981,14, 423. (33) Boykin, D. W.; Baumstark, A. L. Tetrahedron 1989,45,3613. (34) The chemical shifts for (E)- and (27-9could also be affected if the torsional angle between the formyl group and the aromatic ring is significantly different in the two isomers. A torsional angle of 21° was estimatedss for @)-acetanilide. (35) Boykin, D.W.; Deadwyler, G. H.; Baumstark, A. L. Magn. Reson. Chem. 1988,26, 19.
