J. Phys. Chem. 1982, 86,2327-2336
observed frequency in 1,3-DPG corresponds to that for a chain rotational isomer of the 1-chain in DPPC. A summary of the carbonyl frequencies and assignments for the four lipid systems appears in Table I.
Conclusion A consideration of the Raman spectra of the carbonyl stretching mode regions for a series of phospholipids, di-, and triglyceride molecules indicates that the spectral patterns in the 1700-1750-cm-' region are clearly useful in distinguishing between conformations about the C,C&(=O)O-ester groups for the hydrocarbon chains. In particular, the carbonyl stretching modes for acyl chains with an abrupt bend at the carbon 2 position exhibit a lower vibrational frequency in comparison to the carbonyl frequencies for the ester groups of the straight-chain systems. Thus, frequencies for the carbonyl mode from about 1727 to 1744 cm-l reflect a nearly straight chain segment, while frequencies from about 1716 to 1728 cm-' are characteristic of the bent chain conformation. Although some ambiguity may arise for frequencies occurring at 1728 cm-l, the relative intensities of the transitions for the systems investigated, as shown for tripalmitin, allows a confident assignment to be made. Since the
-
2327
carbonyl groups for the sn-1 and sn-2 chains may be separately identified, we have available an important molecular probe for monitoring effects involving changes in bilayer structure at the membrane interface region. Although the carbonyl features broaden, shift, and tend to merge in hydrated systems, curve deconvolution could be used, in principle, to follow effects involving individual chains.' For example, changes in the carbonyl stretching modes resulting from the combined effects of both cholesterol (at low concentrations) and water suggest that the bent sn-2 chain in hydrated DPPC bilayers assumes a conformation more analogous to the linear sn-1 chain.' In a recent infrared study involving the effects of the basic myelin protein on diipalmitoyl phosphatidylglycerol bilayers, we noted that the lipid chain conformations maintain their conformational inequivalence in both the gel and liquid crystalline states.lQ That is, two distinct features at -1726 and 1742 cm-' are observed in the presence of the protein. In the absence of the protein, a broad, nearly symmetrical feature, which is analogous to that recorded for DPPC, is observed -1735 cm-1.237J9 (19)R. G.Adams and I. W. Levin, unpublished data.
Conformers and Internal Rotation Barrlers in Nitrous Acid Esters. Low-Resolution Microwave Spectroscopic Studies Nancy S. True Department of Chemistry, University of California, Davis, Caiifornis 95616
and Robert K. Bohn' Department of Chemistry and Instltute of Materials Science, Universky of Connecticut, Storrs, Connecticut 06268 (Received April 28, 1981; In Final Form: February 3, 1982)
Low-resolutionmicrowave (LRMW) spectra of 10 alkyl nitrites have been observed and characterized. LRMW spectra are sensitive to internal rotation of asymmetric groups only, and these 10 compounds have from one asymmetric internal rotor (about the N-0 bond in tert-butyl nitrite, O=N-O-C(CH,),) to four (about the N - O , O-C, and two subsequent C - C bonds in n-butyl nitrite and isopentyl nitrite). Eight of the ten compounds are observed in conformers having both syn and anti c o d i a t i o n s of the O = N - O - C chain. The two exceptions (anti only) are the two tertiary nitrites in the study which have the C atom bonded to the nitrite group bonded to no H atoms. tert-Butyl nitrite has one stable conformation,isopropyl, neopentyl, isobutyl, tert-amyl, neohexyl, and isopentyl nitrites have two stable conformations, and n-propyl, n-butyl, and 2-butyl nitrites have at least three stable conformations. Most of these have been assigned. Also, eight of the compounds display spectra of species rotating about the 0-C bond above the internal rotation barrier. These species occur only when configuration is anti, and the observations have allowed the low barriers to internal rotation the O=N-0-C above the 0-C bond to be estimated in each case. The two exceptions are tert-butyl nitrite, whose LRMW spectra are insensitive to internal rotation about the 0-C bond, and 2-butyl nitrite.
Introduction Alkyl nitrites are of interest structurally, biologically, and environmentally. The microwave spectrum of methyl nitrite' reveals the presence of syn and anti rotamers, the syn form being more stable by 314 (22) cm-l.ld Barriers (1)(a) W.D.Gwinn, Second Austin Symposium on Gas Phase Molecular Structure, 1968,Abstract M2; (b) P. H. Turner, M. J. Corkill, and A. P. Cox, J.Phys. Chem., 83,1473(1979);(c) K.Endo and Y. Kamura, J. Chem. SOC.Jpn., 729 (1977);(d) P.N.Ghosh, A. Bauder, and HE. H. Gunthard, Chem. Phys., 53,39 (1980). 0022-3654/8212006-2327$0 1.2510
to internal rotation of the methyl group in methyl nitrite are remarkably different, 734 (2) cm-l for the syn formld and 10.1 cm-' for the anti form.lb Recent ab initio Hartree-Fock molecular orbital calculations have reproduced the surprisingly large difference in methyl barriers in the two conformers.2 Three stable rotamers of ethyl nitrite have been identified from microwave spectral data? They (2) F. R. Cordell, J. E. Boggs, and A. Skancke, J. Mol. S t r u t . , 64,57 (1980). (3)P. H.Turner, J. Chem. SOC.,Faraday Trans. 2,75,317 (1979).
0 1982 American Chemical Society
2328
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
a n t i - g a i i c r e ( AG)
an?t - a r t 1 ( A A ) i5 :ONO C)= 180 72(NO C R )
='a@
4(0N3C)= 180"r2(VCC R) -9co
Flgure 1. Conformations of the primary nitrites. Conformations of secondary and tertlary nitrites are similarly defined by allowing R to be the unique substituent and terminal H's to be methyl groups.
--
have the syn-anti ( S A rl(ONOC) = Oo, r2(NOCC)= B O o ) syn-gauche (SG T~(ONOC) = ,'O r2(NOCC) W'), and anti-gauche (AG: T1(ONOC) = 180°, T~(NOCC) 90') structures displayed in Figure 1. Relative energies are 0, 238 (50), and 81 (20) cm-' for the SA, SG and AG conformers, respectively. The energy barrier between the SA and SG forms is estimated as 800 (240) cm-' and the S G SG barrier between 1400 and 4200 cm-'. For anti-ethyl nitrite, the two planar configurations AA and AS are energy maxima. The AA barrier is 137 (7) cm-' and the AS barrier lies in the range 260-1400 cm-l above the AG minima. Other conformational studies of alkyl nitrites, which utilized a variety of experimental techniques, have also demonstrated the presence of syn and anti rotamers. Conclusions regarding conformer assignments, abundance, and barriers to interconversion determined by the various methods are not in agreement. Temperature-dependent lH NMR studies of alkyl nitrites demonstrate the presence of two conformational forms separated by barriers of approximately 10 kcal/moL4-* Spectral assignments are controversial, however. Infrared,gJo and thermodynami~l~ data have also been interpreted in terms of mixtures of syn and anti conformers about T1(ONOC). Infrared spectral assignments of the conformers are not in agreement, and the reported interpretations of the dielectric and thermodynamic data are not unique. The microwave studies of methyl' and ethyl3 nitrites have characterized their structural properties very well. Similar high-resolution microwave studies of large alkyl nitrites are not practical because of high spectral density and (4)H. W.Brown and D. P. Hollis, J. Mol. Spectrosc., 13,305(1976). (5)W.D. Phillips, C. E. Looney, and C. P. Spaeth, J.Mol. Spectrosc., 1, 35 (1957). (6)P. Gray and L. W. Reeves, J. Chem. Phys., 32, 1878 (1960). (7)L. H. Piette and W. A. Anderson, J. Chem. Phys., 30,899(1959). (8)L. H.Piette, J. D. Ray, and R. A. Ogg,Jr., J. Chem. Phys., 26,1341 (1956). (9)P. Klaboe, D.Jones, and E. R. Lippincott, Spectrochim. Acta, Part A , 23,2957 (1967). (IO) P. Tarte, J . Chem. Phys., 20, 1570 (1952). (11)R.J. W. LeFevre, R. K. Pierens, D. V. Radford, and K. D. Steel, A u t . J . Chem., 21, 1965 (1968). (12)P. Gray and M. J. Pearson, Trans. Faraday Soc., 59,347(1963). (13)R. F. Grant, D. W. Davidson, and P. Gray, J. Chem. Phys., 33, 1713 (1960). (14)P. Gray and M. W. T. Pratt, J. Chem. Soc., 3403 (1958).
True and Bohn
complexity. Nevertheless, much structural information is contained in their microwave spectra, even at low resolution; and, even though the spectrum of a single compound cannot be rigorously analyzed, arguments based on analogies among similar compounds can be as powerful in microwave studies as they have been in NMR and infrared studies. This study of n-propyl, isopropyl, n-butyl, 2-butyl, isobutyl (2-methylpropyl), tert-butyl, isopentyl (3methylbutyl), neopentyl (2,2-dimethylpropyl), tert-amyl (1,l-dimethylpropyl), and neohexyl (3,3-dimethylbutyl) nitrites was undertaken to identify and characterize the conformational isomers of primary, secondary, and tertiary nitrites and to determine their relative energies. Since the internal rotation barrier about the 0-C bond is low for alkyl nitrites anti about 71(ONOC), we were able to observe spectra of molecules in free rotor states above this barrier. LRMW spectral data of these vibrationally excited species provide information about internal rotation potential functions.lS2O
Experimental Section Microwave Measurements. Spectra were obtained with samples at pressures of 60-80 mtorr at room and dry-ice temperatures.16 Low-temperature runs were carried out by packing the two X-band sample cells in dry ice. The flanges at the ends of the cells were exposed to the air and kept warmer by fans blowing on them. The average sample temperature was estimated to be -63 f 10 OC. Spectra of n-propyl and n-butyl nitrite were recorded from 18.5 to 38 GHz, those of neopentyl, isobutyl, neohexyl, isopropyl, 2-butyl, tert-butyl, and tert-amyl nitrites from 26.5 to 38.5 GHz, and those of isopentyl nitrite from 18.5to 26.5 GHz. Additional spectra of neopentyl nitrite were obtained by using high-resolution conditions, and the lines of the accidentally symmetric conformer of neopentyl nitrite were measured to a precision of 0.1 MHz. Sample Preparation. Samples of n-propyl, n-butyl, isobutyl, and tert-butyl nitrite were purchased from Pfaltz and Bauer Chemical Co. Isopentyl nitrite was purchased from Eastman Chemical Co. Isopropyl, 2-butyl, neopentyl, tert-amyl, and neohexyl nitrites were synthesized from the corresponding reagent-grade alcohols and sodium nitrite according to the procedure of Noyes.21 The synthesized samples were vacuum distilled before use. All samples were characterized by their infrared and 'H NMR spectra. Gas chromatograms of each sample, using a 6-ft SE-30 column with 5-10-min retention times, showed no detectable impurities. Calculations. Since nitrous acid,22methyl nitrite, and ethyl nitrite3 each display conformations with only planar syn or anti O=NOX (X= H, C) frameworks, we assume that the O=NOC framework is planar (syn or anti) in the 10 alkyl nitrites studied here. Similarly, since only gauche or anti configurations have been observed in substituted alkanes, we assume that only gauche or anti configurations (15) N. S. True and R. K. Bohn, Chem. Phys. Lett., 60,332 (1979). (16)L. P.Thomas, N. S. True, and R. K. Bohn, J.Phys. Chem., 84, 1785 (1980). (17)N. S.True, C. J. Silvia, and R. K. Bohn, J.Phys. Chem., 85,1132 (1981). (18)N. S. True, M. S. Farag, J. Radhakrishnan, and R. K. Bohn, Spectrochim. Acta, submitted for publication. (19)N. S.True, M. S. Farag, R. K. Bohn, M. A. MacGregor, and J. Radhakrishnan, Spectrochim. Acta, submitted for publication. (20)N. S. True, R. K. Bohn, A. Chieffalo, and J. Radhakrishnan, Spectrochim. Acta, submitted for publication. (21)W. A. Noyea, 'Organic Synthesis", Collect. Vol. 11, E. C. Honning, Ed., Wiley, New York, 1943,p 108. (22)A. P. Cox, A. H. Brittain, and D. J. Finnigan, Trans. Faraday SOC.,67,2179 (1971).
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982 2329
Conformers In Nitrous AcM Esters
TABLE I: Structural Parameters Assumed for Alkyl Nitrites r( 0 - N ) 1.182 A" r(N-0) 1.398 A" 1.431 A" r(0-R) LONO LNOR r(C-C) r(C-H) LOCC LCCC L CCH r(C-C) r(C-H) LOCC LCCC LCCH
114.8"" 114.90a 1.528 A b 1.09 a 109.5' 114" 109.5' 1.528 1.09 a 109.5' 109.5" 109.5'
r(C-C) r(C-H) LOC,C, LClCZC,
LCCH
1.528 Aa 1.09 a 109.5" 114" 112" 109.5" 1.528 A b 1.09 a 109.5' 112" 109.5'
r(C-C) r(C-H) LOCC L C2C I C4 LCCH
1.528 ' . 4 1.09 A 112" 109.5" 109.5"
Lc2c3c4
LCCH r(C-C) r(C-H) LOCC Lc2c103
'>$i 1 1C141H3
I
C i 2 ) ti3
Ci5iH3
a
Reference l b .
Reference 3.
r(O=N) r ( N - 0 )' r( 0-R) LONO LNOR r(C-C) r(C-H)
o"-O \R
O\
HJ.C-C-H
O\
v,c
H'
\ 1 )-
H,'
LCCC LCCH r(C-C) r(C-H) LOC,C,
1.164 A" 1.415 A" 1.436 A" 111.8"a 112.9"" 1.528 Aa 1.09 A 109.5' 112" 109.5" 1.528 A' 1.09 a 109.5"
Lc1c2c3
114"
L
A. --
c 12 I ti Cc,\
13)-
H'
C 14 I h 3
occ
Lc2c3c4
LCCH r(C-C) r( 0 - H ) LOCC Lc1c2c3
Lc2c3c4
LCCH r(C-C) r(C-H) LOCC Lc1c2c3
Lc2c1c4
LCCH r(C-C) r(C-H) LOCC LC,C,C, tEgAC3
'
114" 109.5" 1.528 A' 1.09 A 109.5"' 114" 109.5" 109.5" 1.528 A b 1.09 a 109.5' 114' 112" 109.5" 1.528 A C 1.09 A 112" 114" 109.5' 109.5'
S. Fitzwater and L. S. Bartell, J. Mol. Struct., 37, 1 1 3 (1977).
are present in the alkyl groups of the 10 compounds in this study. Therefore, for each nitrite, B + C was calculated as a function of T~(NOCC) with T~(ONOC) fixed in syn or anti configurations and any subsequent C-C torsional angles fiied in gauche or anti configurations. Table I lists the geometrical parameters used. Assuming that the bond lengths are accurate on the average to k0.02 A and the bond angles to k2O, the calculated B + C values are accurate to --f2%. The B C functions were calculated by assuming rigid rotation about the C-0 bond; none of the other parameters was allowed to relax. For n-propyl and neopentyl nitrites, wave functions and energy levels of the 0-C torsion were calculated.Ba The internal rotation constants required for those calculations were obtained by using Polo's method.Bb Expectation values of B + C were then calculated by using the torsional wave functions and the B + C functions shown in the figures. Barriers to internal rotation about the C-0 bond in the other alkyl nitrites were estimated by comparing the observed relative energy of the group of free rotor states to values calculated238for various barrier heights.
+
Results Conformational assignment of LRMW band series is primarily based on matching observed B + C values with those calculated from models. Also, band shape, bandwidth, vibrational satellite patterns, relative intensities and stabilities, and comparison with analogous compounds may add significant evidence. (23)(a) J. D.Lewis, J. B. Malloy, Jr., T. H. Chao,and J. h e , J . Mol. Struct., 12,427 (1972); (b) S.R.Polo, J . Chem. Phys., 24,1133(1956). The computer program CART,written by H. M. Pickett, calculates Gii-l for trial structures.
One Asymmetric Internal Rotor (about N-0 Bond) tert-Butyl Nitrite. tert-Butyl nitrite displays a single R-branch a-type series (frequency = (J+ 1)(B + C)) with B C = 3440.2 (5) MHz and a single b-type series (frequency = (K11/2)(2A - B - C))%with 2A - B - C = 5343 (10) MHz. For syn-tert-butyl nitrite B C and 2A - B - C are calculated to be 4355 and 4316 MHz, respectively, and for anti-tert-butyl nitrite 3467 and 5730 MHz, respectively, using parameters given in Table I. Comparison of observed and calculated values shows that both band series are due to the same conformational isomer which has a planar or nearly planar anti O=N-0-C configuration. The relative intensities of the two band series do not change with temperature, which is consistent with their assignment to the same species. The narrow a-type bands (100 MHz) are consistent with an anti isomer which is a nearly symmetric rotor. Calculated values of K ( K = (2B - A - C)/(A- C)) are -0.93 and -0.99 for syn and anti, respectively. Since the tert-butyl group has 3-fold symmetry, the molecular rotational constants are independent of that group's orientation.
+
+
+
Two Asymmetric Internal Rotors (about N-0 and 0 - C Bonds) Neopentyl Nitrite. Neopentyl nitrite is completely analogous to ethyl nitrite except that each terminal H atom in ethyl nitrite has been replaced by a methyl group. LRMW R-band spectra of neopentyl nitrite at 21 and --63 "C are shown in Figure 2, a and b, respectively. R-branch a-type band series associated with three species are present. B + C values are 2231.2 (3), 2195.7 (7), and (24)W.E.Steinmetz, J. Am. Chem. SOC.,96,685 (1974).
2330 The Journal of Physical Chemistty, Vol. 86, No. 13, 1982 I
I
II
I
AFr
I
I
I
1
1
.
True and Bohn
neopentyl nit r i t e
1
30GiiZ
35 I
b 1
1
1
1
l
l
1
1
l
l
30 GPz
35
I
AFr
I
+
Flgure 3. Observed and calculated B C values for neopentyl nitrite. The lower mild curve displays calculated B C values as a function of r2(NOCC)for 7,(0=NOC) = 180' (A). The upper dashed curve displays calculated 8 C values as a function of r2(NOCC)for 7,(0=NOCC) = 0' (S). The closed circles indicate the assigned configurations. The error bars correspond to estimated 2% uncertainty C due to uncertainties in the assumed structural parameters. in B i-
+
+
Flgure 2. LRMW spectra of neopentyl nitrite: (a) R-band spectrum at 21 'C swept at 10 MHz/s with a 1-s time constant. The band markers are calculated from (J 1x6 C ) with B C = 2231,2196, and 2161 MHz for SA, AG, and GG conformers, respectively. (b) Same as part a except temperature is - 4 3 'C.
+
+
+
2161.1 (3) MHz for series labeled SA, AG, and AFr (anti-free rotor), respectively. The relative intensities of the three band series vary dramatically from 22 "C, 1:3.3:75, to --63 "C, 1:1.2:7, for the SA, AG, and AFr series, respectively. Band area measurements1' yield the relative energies 0,1.2 (3), and 2.0 (4) kcal/mol for the SA, AG, and AFr species, respectively. Spectral data appear in Tables I1 and 111. The three band series of neopentyl nitrite have similar B C valuea and are compatible with a range of geometries (Figure 3) including the syn-anti (SA; rl(ONOC) = ,'O TZ(NOCC)= 180") and anti-gauche (AG; T1(ONOC) = 180°, T~(NOCC) 90') conformers observed for ethyl nitrite. Variations in bandwidths and relative energy data allow assignment of the two more stable conformers of neopentyl nitrite. The calculated value of K for the AG (T1(ONOC) = 180°, r2(NOCC) 90-120') conformer of neopentyl nitrite is -0.99 (1). Bands of the observed AG series are so narrow, 2 MHz wide, that they are compatible only with a symmetric or very nearly symmetric top geometry. The SA conformer of neopentyl nitrite has a calculated K value of -0.96 (1) appropriate for bands approximately 100 MHz wide. The AG and SA band series of neopentyl nitrite are therefore assigned to anti-gauche (rl(ONOC) = 180°, r2(NOCC) 110") and syn-anti (r2(ONOC) =,'O T~(NOCC) = 180") structures, respectively, identical with the two most stable conformers in ethyl nitrite characterized by high-resolution microwave spectroscopy? The highest-energy conformer observed in ethyl nitrite has a syn-gauche conformation. The corresponding conformer in neopentyl nitrite would have the nitrite and tert-butyl (instead of methyl) groups in a gauche configuration and is not observed. Since the AG species is accidentally a nearly symmetric top, many resolved vibrational satellites are present in its LRMW spectrum. Figure 4 displays the spectrum of
+
-
-
-
0 0 0
1
1
2
2
3
2 ' 3
1
0
"
0
0
4 0-2 torslor 0 0 -JO t o r s or
Figure 4. LRMW spectrum of neopentyl nitrite from 36.4 to 37.5 GHz recorded at - 4 3 "C. The bands correspond to the J 1 J , 17 16 transkion of the AG and AFr species. Vibrational quantum numbers label the satelikes of the AG band. (a) Stark field is 3200 Vlcm. (b) Stark field is 600 V/cm.
-
+
- -
+-
neopentyl nitrite between 36.4 and 37.5 GHz which contains the J + 1 J , 17 16 pileups of the species labeled AFr and AG. Spectra were recorded with 3200 V/cm (Figure 4a) and 600 V/cm (Figure 4b) Stark fields. Displaced to lower frequencies from the ground-state band of the AG conformer is a series of satellite bands attributed to successively excited states of the T,(NOCC) torsion, based on analogy to AG ethyl nitrite.3 States up to u = 4 have been observed. The assignments are indicated in Figure 4. The u = 3 satellite is superimposed upon the
The Journal of Physical Chetnistty, Vol. 86, No. 13, 1982 2331
Conformers in Nitrous Acid Esters
TABLE 11: Rotational Constants, Band Intensities, and Relative Energies of the Conformers of Alkyl Nitrites tert-Butyl Nitrite obsd conformer B t C, MHz A , MHz av bandwidth, fwhm, MHz obsd conformers B + C, MHz u = (0,O)
A 3440.2 ( 5 ) 4392 ( 5 ) 100
Neopentyl Nitrite SA AG
AFr
2231.2 ( 3 ) 2195.67 ( 7 ) 2161.1 ( 3 ) 2181.74 (8) 2168.07 ( 5 ) u = (3,O) 2156.9 ( 2 ) u = (4,O) 2147.6 ( 3 ) u = (5,O) 2140.6 ( 2 ) u = (0,l) 2199.8 (1) u = (0,2) 2203.7 ( 2 ) u = (1,l) 2187.0 (1) u = (2,l) 2173.3 (1) relative intensity, 1 3.3 75 22 "C relative intensity, 1 1.2 7 --63 "C relative energy, 0 1.2 ( 3 ) 2.0 ( 4 ) kcalimol av bandwidth, 100 2 125 fwhm, MHz
obsd species se AG AFr B t C, MHz 4779 ( 6 ) 3954 ( 4 ) 4003 ( 3 ) relative intensity, 21 "C 1 2 10 2 4 relative intensity, --63 "C 1 relative energy, kcal/mol 0 -0 -1 av bandwidth, fwhm, MHz 100 100 300 n-Propyl Nitrite SAG (or obsd conformers SAA SGA) AGA AFrA B + C, MHz u=o 3138 (1) 3535 ( 2 ) 2936 (1) 2864 ( 2 ) u= 1 2899 (1) relative intensity, 1 1.3 0.6 5.6 25 "C (0.3, u = 1)
0
-0
0.8 3 (0.2, u = 1) -0 -1
75
100
75
1.0
relative energy, kcal/mol av bandwidth, fwhm, MHz obsd species B + C, MHz
Neohexyl Nitrite SAA AG A
1
-1
8
0
-0
-1
100
50
150
2-But 1 Nitrite
Isopropyl Nitrite
--
AFrG
2582.6 ( 5 ) 2485.9 ( 3 ) 2455.9 ( 3 ) 2480 (1) 2500 ( 2 ) 2522 ( 2 ) 1 -1 20
--
u = (1,O) u = (2,O)
relative intensity, 1 63 "C
Isobutvl Nitrite SAC': Aee
obsd species B t C, MHz u = (0,O) u = (1,O) u = (0,l) u = (0,2) relative intensity, 25 "C relative intensity, 6 3 "C relative energy, kcal/mol av bandwidth, fwhm, MHz
250
obsd species B t C, MHz 2C, MHz relative intensity, 21 "C relative intensity, --63 "C relative energy, kcalimol av bandwidth, fwhm, MHz
1385.6 ( 7 ) 1315.4 ( 3 ) 1304.6 ( 2 ) 1312.0 ( 5 ) relative intensity, 1 15 25 "C relative intensity, 1 0.3 6 --63 " C relative energy, 0 -0 -1 kcalimol av bandwidth, 150 50 100 fwhm, MHz u=o u= 1
1
1
0.5
0
-0.5
0
400
300
303
tert-Amyl Nitrite obsd conformers AAA AGA AFrA B + C, MHz 2278 (1) 2474 (1) 2403 (1) relative intensity, 21 "C 1 1 2 relative intensity, --63 "C 1 1 0.5 relative energy, kcal/mol 0 -0 -1.5 av bandwidth, fwhm, MHz 75 150 300
obsd species B t C. MHz
n-Butyl Nitrite SAGAa (or SAAA SGAA)
1838.5 2058 (7) u= 1 2063 u= 2 2066 u= 3 2070 u=4 2075 relative intensity, 1 0.5 21 "C relative intensity, 1 0.5 63 "C relative energy, 0 -0 kcallmol av bandwidth, 100 50 fwhm, MHz u=o
--
AFrA
not AG A AGGa assigned SG Aa 3285 (4 2580 (1) 2875 ( 3 ) 2625 (1) 1 2 0.5
AGAA
AFrAA
(1) 1789 ( 3 ) 1773.8 ( 51 (2) (2) (2) (2) 6
0.3
4
-0
-0.5
100
175
Isopentyl Ni ;rite obsd species SAAG AGAG AFrAG B + C, MHz 1564 (1) 1642 (1) 1482 (1) relative intensity, 21 "C 20 relative intensity, --63 "C 1 0.3 20 relative energy, kcal/mol 0 -0 >O av bandwidth, fwhm, MHz 100 100 100
* Tentative assignment.
- -
s t r o n g amorphous absorbance corresponding to the J
+
1 J,17 16 transition of the AFr series. At lower Stark fields (Figure 4b) the intensity of the anomalous AFr band diminishes and the u = 3 satellite of the AG conformation is more clearly identified. This pattern is reproducible for each J + 1 J transition. Since the AFr/AG intensity ratio increases monotonically up to a Stark field of 3200 V/cm, the maximum available, the reported intensity ratio is only a lower limit. B C varies smoothly as a function
+
of vibrational quantum number for satellites of the 0-R torsional mode of anti-neopentyl nitrite. Relative intensity measurements of these satellites yield a torsional frequency of 40 (20) cm-'. A striking feature of the LRMW spectrum of neopentyl nitrite is the AFr band series' large intensity which is even greater than indicated in Table I1 because of incomplete Stark modulation of the AFr bands. A simple model featuring a low internal rotation barrier
2332
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
True and Bohn
TABLE 111: B t C Values, Energies, and Intensities of Satellitesin the 0-Alkyl Torsion of Neopentyl Nitrite obsd
calcda int,
E, cm-'
BtC AG
u= 0 u= 1 u=
2
u= 3 u=
4
v=5 u= 6
Fr
2195.67 0 (7) 2181.74 40 (8) 2168.07 80 (51 2156.9 ( 2 ) 2 1 4 7 . 6 ( 3 ) b 300 2 1 4 0 . 6 ( 2 ) 300
ethyl nitrite conformers, the configuration about T~ in isopropyl nitrite is probably g a u ~ h e . ~ ~ - ~ ~ It is clear from calculated B + C values that the 3954and 4003-MHz series derive from species anti about T ~ It . is also clear that these two species cannot both represent stable conformations since they correspond to structures differing only by about 10" rotation about 72. The less intense band series, 3954 MHz, must be the stable AG conformer because it lies 1 kcal/mol lower in energy than the 4003-MHz species (Table 11). The broad, intense, high-energy species with B + C = 4003 MHz is assigned to the AFr (anti-free rotor) species torsionally excited above the barrier about T ~ The . AG conformer is unambiguously gauche about T~ since an anti form would be displaced to a much smaller B C value relative to the free rotor form. From the temperature dependence of the relative intensities and the value of the relative intensities, the barrier to internal rotation about T~ is estimated to be 0.5 kcal/mol.
25 "C
int, E, 25 B t C cm-' "C
1 2218.4
0
(20)
2206.6
20
(40)
2196.5
40
2187.2 2177.9
58 76
(100) (100)
2166.2 93 2159.1' 108 2 1 4 2 . 4 109 2 1 6 1 . 1 ( 3 ) 7 0 0 ( 2 0 0 ) 75 2 2 0 3 300
1
-
+
5
a V(~)=Z:~(Vn/2)(1-cosn~);V,=-15cm V,= -~, - 1 5 0 cm-', V,= - 2 2 cm-I, V = -10 cm-'. B + C ( T )= 2470 - 2 7 5 ( 1 - cos T ) MHz. Apparently split by perturbation. ' Split by double potential well.
about T~(NOCC) for anti-neopentyl nitrite can explain most of the spectral features. A torsional potential function which is consistent with the observation but is probably not unique is v(T2)
= c ( v n / 2 ) (1 - cos n72) n
where VI = -15, V2 = -150, V, = -22, and V, = -10 cm-'. This potential function has minima at *85" and barriers of 170 and 133 cm-l at T~ = 0" and T~ = 180°, respectively. Expectation values of B C calculated by using this potential function, an internal rotation constant of 0.85 cm-', MHz qualitatively and B + C ( T ~=) 2470 - 275(1- cos agree with observed values (Table 111) for excited torsional states. No absorbance attributable to an SG (syn-gauche) conformer of neopentyl nitrite similar to that observed in ethyl nitrite has been found. This is not unreasonable since the tert-butyl group is so much bulkier than a methyl group and the conformer is calculated to be less polar and prolate ( K -0.88) than the observed species. Isopropyl Nitrite. Isopropyl nitrite is similar to ethyl nitrite since it has a carbon atom bonded to two identical substituents, H atoms for ethyl and methyl groups for isopropyl. LRMW band spectra of isopropyl nitrite display three band series. B + C values are 4779 (6), 3954 (4), and 4003 (3) MHz for conformers designated SG, AG, and AFr, respectively. Temperature-dependent intensity data shown in Table I1 demonstrate that the SG and AG species have approximately equal energy and the AFr form has -1 kcal/mol more energy. Bands of the SG and AG species are much narrower than bands of the AFr series and all are structured. Numerous weak narrow absorbances which do not index to integers and appear to be strong lines are present in LRMW spectra of isopropyl nitrite at both 21 and --63 "C. In isopropyl nitrite the species can be assigned with considerable rigor since the forms syn about T' have B + C values very different from the anti forms (see paragraph at end of text regarding supplementary material). The 4779-MHz series is unequivocally assignable to the species which is syn about T ~ Whether T ~ ( N O C His) nonzero (gauche) or zero (NOCH syn) cannot be directly determined by LRMW spectroscopy. However, since ethyl nitrite has stable syn-anti and syn-gauche conformers, and since the methyl groups in syn-gauche isopropyl nitrite simultaneously occupy positions very similar to the methyl positions in the stable
+
-
.
Three Asymmetric Internal Rotors (about N-0, 0-C, and C-C Bonds) n-Propyl Nitrite. Four R-branch a-type band series are present in LRMW spectra of n-propyl nitrite with one of them displaying a resolved vibrational satellite. B + C values are 3138 (l),3535 (2), 2936 (l),(2899 (l),u = 1) and 2864 (2) MHz for series designated SAA, SAG, and AFrA, respectively. The relative intensity ratios at 25 "C and at w-63 "C (Table 11) indicate that the SAA, SAG, and AGA species are equally stable. The AFrA species and the vibrationally excited AGA species have approximately 1 and 0.5 kcal/mol more energy, respectively. Comparison of observed and calculated B + C values (supplementary material) allows firm assignments of some of the species. The stable species with B + C = 2936 MHz agrees only with the calculated value for an anti-gauche-anti configuration of T ~T, ~ and , T ~ . Also, the narrow (75 MHz) bandwidth is consistent with a nearly symmetric rotor ( K = 4 . 9 9 , calculated), the related compound, ethyl nitrite, also has a stable conformer with T1,72 anti-gauche, and the observed shift of B + C with vibrational excitation (as) well with that calculated signed to torsion about T ~ agrees from a reasonable internal rotation potential function.15 The 3138-MHz B + C value is consistent with either a syn-anti-anti (SAA) or anti-anti-gauche (AAG) conformer. Since an anti-anti configuration about 71,72 is not observed in ethyl nitrite or in any other of the nitrites studied here, we discard the AAG assignment. On the other hand, the SA form is stable in ethyl nitrite and it is observed in all of the other primary and secondary alkyl nitrites included in this study. Therefore, the 3138-MHz band series is assigned to the SAA conformer. The 3535-MHz species is the most ambiguous to assign. The B + C value is compatible with calculated values for five conformers, SAG, SGA, ASA, AGG, and AGG (supplementary material). The most probable assignment is the syn-anti-gauche (SAG) conformation which is consistent with the presence of the SAA species, the SA species observed in ethyl nitrite, and other SA species observed in related compounds in this study. The remaining broad (250 MHz) and intense 2864-MHz band series is the most interesting. Its B + C value is consistent with an AGA conformation with T~ increased by 10" from the more stable 2936-MHz species. The (25)N. S.True and R. K. Bohn, J. Mol. Struct., 36, 173 (1977). S.True and R. K. Bohn, J . Mol. Struct., 50, 217 (1978). (26),N. (27)C.J. Silvia, N. S.True, and R. K. Bohn, J . Mol. Struct., 51, 163 (1979).
The Journal of
Conformers in Nitrous Acid Esters
presence of a vibrational satellite of the 2936-MHz species with B C = 2899 MHz and the very large intensity of the 2864-MHz bands rule out the possibility that the 2864-MHz species is due to a stable conformer. The r1 internal rotation barrier in nitrites is quite high, -10 kcal/mol based on DNMR data.4-8 The B + C value, energy, and intensity of the 2864-MHz series is compatible, however, with AGA species torsionally excited about the ~ ~ ( 0 -bond c ) above the potential barrierlSz0 and designated anti-free rotor-anti, AFrA. Ethyl nitrite at high Stark fields displays a similar high-energy, structured band series, which has a B + C value coincident with the u = 2 satellite of the r2(NOCC) torsion of the AG conformer. The intensity of this band is greater than the intensity of the AG ground-state band but is not nearly as great as in the case of n-propyl nitrite. Neohexyl Nitrite (3,3-DimethylbutylNitrite). Neohexyl nitrite is s i m i i to n-propyl nitrite except that the terminal methyl group has been replaced by a tert-butyl group. LRMW band spectra of neohexyl nitrite at 21 and - 4 3 'C are dominated by an R-branch a-type band series having a B C value of 1304.6 (2) MHz designated AFrA. Two weaker series designated SAA and AGA have B C values of 1385.6 (7) and 1315.4 (3) MHz. At room temperature, bands of the AGA conformer are lost in the spectral noise. The variation of intensities with temperature (Table 11) indicates that the AFrA species is -1 kcal/mol higher in energy than the SAA conformer and that the AGA conformer is probably the most stable species. B + C values were calculated for neohexyl nitrite (supplementary material) as described above for neopentyl nitrite. The narrow 1315.4-MHz band series with its vibrational satellite at 1312 MHz can be confidently assigned to the stable anti-gauche-anti (AGA) conformer because of the agreement between calculated and observed B + C values, the consistency between the narrow bands (50-MHz fwhm) and the very symmetric structure ( K = -0.99, calculated), and the low-frequency displacement of the 0-C torsionally excited satellite which is also observed in the AG conformer of ethyl nitrite: AG neopentyl nitrite, AGA n-propyl nitrite, and AGG isobutyl nitrite (vide infra). The 1385.6-MHz series is then assigned to the syn-anti-anti (SAA) conformation, the only other conformation with calculated B + C values compatible with the observed. This assignment is supported by the precedents of the SA conformers of ethyl nitrite3 and neopentyl nitrite, the S A 4 form of n-propyl nitrite, and the corresponding SA... conformers observed in all of the remaining primary alkyl nitrites included in this study. The extremely intense 1304.6-MHz species is consistent with those species anti about T~ and 73 and torsionally excited above the low internal rotation barrier about T~ (anti-free rotor-anti, AFrA). This series consists of the band heads of the rotational spectra of these torsionally excited species. The average relative energy of these species, 1 kcal/mol, is consistent with a torsional barrier of -0.5 kcal/mol. Isobutyl Nitrite (2-Methylpropyl Nitrite). LRMW band spectra of isobutyl nitrite at 21 and - 4 3 'C display R-branch a-type band series consistent with three species. B + C values are 2582.6 (5), 2485.9 (3), and 2455.9 (3) MHz for species designated SAG, AGG, and AFrG, respectively. The temperature-dependent intensity data, which appear in Table 11, indicate that the SAG and AGG forms have equal energy and the AFrG species has approximately 1 kcal/mol more energy. To characterize the rotamers of isobutyl nitrite, we calculated B + C as a function of the three torsional angles rl(ONOC), 7,(NOCC), and r3-
+
+
+
-
Physlcal Chemistry, Vol. 86, No. 13, 7982 2333
(OCCH) (supplementary material). The two most stable conformers are most reasonably assigned to AGG (rl(ONOC) = 180°, T~(NOCC) l l O o , T~(OCCH) 60') and SAG (rl(ONOC) = ,'O 72(NOCC) = 180°, T~(OCCH) 60') configurations. The B C value of the SAG series is also compatible with an AAA structure. This assignment is unlikely since T~ = 180°, 72 = 180' structures have not been observed in ethyl and n-propyl nitrites whose conformations are better characterized and T~(OCCH)= 180' structures are sterically unfavorable. The AGG series of isobutyl nitrite has resolved vibrational satellite bands displaced to lower frequencies from the ground-state bands. Similarly displaced satellites have been observed for analogous AG conformations of ethyl, neopentyl, and n-propyl nitrite. It also displays a series of higher-frequency satellites which could arise from torsion about T~(OCCH). The highest-energy but very intense 2455.9-MHz species is assigned to torsionally excited AGG species in states above the T~ barrier with T~ = MOO, 73 = 60' and is designated AFrG (anti-free rotor-gauche). The relative energy of this series, 1 kcal/mol, is consistent with a -0.5 kcal/mol internal rotation barrier. 2-Butyl Nitrite. LRMW band spectra of 2-butyl nitrite display three R-branch a-type band series with B C values 3285 (4), 2580 (l),and 2975 (3) MHz. There is a fourth band series whose frequencies are given by a constant times 0.44 plus an integer. This is probably a type I1 seriesz8with spacing 2C which is 2625 (1) MHz. Only one of the band series can be confidently assigned. The 2580-MHz series is only consistent with the calculated B + C value of the anti-gaucheanti (AGA) conformer where the gauche designation about 72 describes the OCCH dihedral angle. The more compact 2875- and 3285-MHz species are each consistent with calculated B C values of four or more configurations and cannot be assigned with confidence (supplementary material). On the basis of analogies to other compounds in this study, we suggest that the 2875-MHz series is due to the AGG conformer and the 3285-MHz series is due to the SGA conformer. The 2625-MHz series, which obeys type I1 R-branch a-type formalism,28 also cannot be confidently assigned. A 2625-MHz value for 2C is only consistent with species anti about 71on the basis of our calculations and may be due to one of the species already displaying a normal R-branch a-type band series or it may be caused by a fourth species in the vaporaB The conformational assignments of 2-butyl nitrite are the least definitive of any reported in this study, but it is clear that there are at least three conformational isomers present in the gas phase and they all have similar energies. tert-Amyl Nitrite (1,l-Dimethylpropyl Nitrite). The LRMW spectrum of tert-amyl nitrite (analogous to npropyl nitrite with two a-methyl groups or tert-butyl nitrite with an added terminal methyl group) displays three R-branch a-type band series. B C values of 2278 (l), 2474 (l),and 2403 (1) MHz are obtained for series labeled AAA, AGA, and AFrA, respectively. The temperature variation of the bands' intensities reveals that the AAA and AGA conformers have approximately equal energy and the AFrA species has approximately 1.5 kcal/mol more energy. Calculated B C values using assumed structural parameters are compatible with those observed only for structures in which both T~ and 73 have anti configurations.
-
-
-
+
-
+
+
+
+
(28) S. J. Borchert, J. Mol. Spectrosc., 57, 312 (1975). (29) E. M. Bellott, Jr., Ph.D. Thesis, Harvard University, Cambridge, MA, 1976.
2334
The Journal of Physical Chemistty, Vol. 86, No. 13, 1982
The calculated B + C values for other configurations of 7 1 and 73 are all considerably larger than the values observed. The calculations also show that many of these other configurations are too asymmetric to yield band spectra. The cluttered appearance of the spectrum with many weak bands not assignable to band series suggests that other species are present in addition to the three displaying a-type band series. The sharp 2278-MHz band series is assigned to the all-anti structure, AAA, because of its agreement with calculated values ( B + C = 2289 MHz, K = -0.99). A rotation about 7 2 of 120° increases the B + C value about 200 MHz and diminishes the symmetry, predictions precisely consistent with the observed 2474-MHz series which is thereby assigned AGA. The relatively intense, broad, higher-energy 2403-MHz species is consistent with species freely rotating about 7 2 above the internal rotation barrier which is estimated to be 0.8 kcal/mol.
Four Asymmetric Internal Rotors (about N-0, 0-C, C-C, and C-C Bonds) N-Butyl Nitrite. LRMW spectra of n-butyl nitrite at 21 and -433 "C display four a-type band series. Species designated SAAA, SAGA, AGAA, and AFrAA have B + C values 1838.5 (7), 2058 (l),1789 (3))and 1773.8 (5) MHz, respectively. The temperature dependence of the intensities indicates that the AFrAA conformer has approximately 0.5 kcal/mol more energy than the SAAA and SAGA species. Restricting consideration to conformers which have either syn or anti configurations about 7 1 coupled with only gauche or anti configurations about 72, T ~ and , 74 still allows 34 spectroscopically distinguishable conformers. Some are sterically unfavorable and some are too asymmetric to yield LRMW band spectra. In spite of this complexity, several conclusions can be reached and a plausible assignment of the four observed band series can be made. Calculated B + C values (supplementary material) of species with two gauche angles (with the exception of an anti-gauche-anti-gauche, AGAG, form) are much larger than those observed. By analogy with ethyl and n-propyl nitrite, stable SAAA and AGAA forms are expected and calculations show that B + C values for these two conformers should be similar. Also these two forms have equal calculated asymmetry parameter values ( K = -4.98 f 0.02) and should have similar narrow bandwidths. The 1838.5- and 1789-MHz series fulfill all of these criteria and are assigned to the SAAA and AGAA conformers, respectively. The calculated B + C values of AAAG also fall into the same region, but, since conformers with 71 and 7 2 both anti are not observed in ethyl or n-propyl nitrite, none is invoked for n-butyl nitrite. The 2058-MHz band series with its associated vibrational satellites (v = 0-4) is consistent with B + C values of SAGA, AAGA, AGAG, or SGAA conformers (supplementary material). The AGAG assignment is unlikely since its calculated asymmetry parameter value, -0.93, is inconsistent with the narrow width, 50 MHz, of the observed bands. The AAGA form is unlikely because conformations anti about both 7 1 and 7 2 have not been observed in similar compounds. The B + C value, narrow bandwidth, and displacement of the vibrational satellites to higher frequency are all consistent with the SAGA assignment. The excited mode is most likely the torsion about 7 2 . The SGAA assignment is also possible. The most intense band series (B + C = 1773.8 MHz), which species has -0.5 kcal/mol more energy than the other forms, remains to be assigned. Its near coincidence with the 1789-MHz AGAA series and its large intensity suggest that it is due to those species freely rotating about
True and Bohn
the barrier to internal rotation about r2. The bands' temperature variations are in reasonable agreement with a barrier of 0.5 kcal/mol about 72, the same value reported above for the AFrA species of n-propyl nitrite. Isopentyl Nitrite (3-Methylbutyl Nitrite). Isopentyl nitrite produces three LRMW R-branch a-type band series having B + C values of 1564 (l),1642 (l),and 1482 (1) MHz designated SAAG, AGAG, and AFrAG, respectively. Intensity data (Table I) indicate that the SAAG and AGAG species are more stable than the very intense AFrAG species. The conformational possibilities for isopentyl nitrite are as numerous as for n-butyl nitrite. Stable forms with syn-anti and anti-gauche configurations about r1 and 7 2 are expected. Calculated values of B + C for the various combinations of syn and anti configurations about 71 coupled with gauche and anti configurations about 73 and 74 (note that 74 labels the CCCH torsion) were calculated (supplementary material). SAAG and AGAG forms have nearly equal calculated B + C values and agree well with the observed 1642- and 1564-MHz values and are so assigned. B C values of the AGAA form also fall in the correct region but are less likely because of the sterically unfavorable anti (CCCH) configuration about 7+ The most intense band series, 1482 MHz, is due to a species of higher energy and has a B + C value consistent with the species rotating above a low internal rotation barrier about 7 2 coupled with an anti configuration about 71 and an anti and gauche configuration about 73 and 74, respectively (AFrAG). From the relative intensities of the bands at dry-ice temperature, the barrier about 7 2 is estimated to be -0.5 kcal/mol.
+
Discussion The only compound in this study with just one asymmetric internal rotor is tert-butyl nitrite. It displays only an anti conformation about T~(ONOC).In the structurally similar methyl nitrite' both syn and anti conformers are observed, and the syn is more stable by 314 (22) cm-'. In ethyl nitrite3 the more stable syn form is more stable than the anti form by only 81 (2) cm-l. In this work we report that the syn and anti forms of isopropyl nitrite are about equally stable. Our results indicate that the substitution of three methyl groups into methyl nitrite (to yield tertbutyl nitrite) makes the syn form sufficiently unstable that it is not observed. In the other tertiary nitrite studied in this work, tert-amyl nitrite, only conformers which have anti configurations about T~(ONOC) are observed. The steric repulsion between the terminal oxygen atom and a tertiary alkyl group is apparently sufficient to make the anti form much more stable than the syn. Low-resolution microwave data give no information about the torsion of the symmetric tert-butyl group. The two alkyl nitrites studied with two asymmetric internal rotors, neopentyl and isopropyl nitrites, each display a single stable conformer with T~(ONOC) syn and one with 71 anti. In addition, each displays a band series assigned to the species with 7 1 anti but excited above a low internal rotation barrier about T~(NOCC).The SA conformer of neopentyl nitrite is sterically sensible since the tert-butyl and nitrite groups are maximally separated. An SG conformer would bring the two groups much closer together and is not observed. The 71 syn form of isopropyl nitrite, SG, has the terminal methyl groups in syn-anti and syngauche configurations, respectively. These correspond precisely to stable configurations in ethyl nitrite.3 Our microwave data do not rule out the possibility that r2(NOCH) may be anti instead of gauche. In such a case both terminal methyl groups would be in syn-gauche configurations. The microwave data do clearly show that
Conformers in Nitrous Acid Esters
there is only one conformer in isopropyl nitrite with T ~ (ONOC) syn, however. Neopentyl and isopropyl nitrites also each display a single stable conformer with T1(ONOC) anti. In neopentyl nitrite the tert-butyl group lies in the gauche T~(NOCC)configuration, not the anti. This is consistent with ethyl nitrite3 which also does not have a stable anti-anti conformation but only a stable anti-gauche form. Similarly, none of the other five nitrites of primary alcohols studied here, n-propyl, n-butyl, neohexyl, isopentyl, and isobutyl nitrites, displays a stable anti-anti conformer. Cordell, Boggs, and Skancke2 have explained that the low methyl torsion barrier in anti(.rl) 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 and T~ is anti. Their model is consistent with our results on nitrites of primary alcohols since the stabilization of a primary C-H bond in an eclipsed configuration places the C-C bond in a gauche configuration as observed. If one extends the model to predict stable conformers of nitrites of secondary alcohols, a stable anti-syn form would be indicated, but we have determined that in isopropyl nitrite 72(NOCH) is gauche, not syn. If one of the stable species in isopropyl nitrite were an anti-syn conformer, then the intense species freely rotating about T ~ would , have a B + C value about 400 MHz below that of the SA species, which is not consistent with the observations. If our assumed structural parameters are sufficiently inaccurate that the 4779-MHz species were the SA species, then the 3954-MHz species would have to be an AA form, completely at odds with the model. Also, the intense 4003-MHz series would not be explained. Our SG and AG assignment of the two stable species requires that the methine hydrogen atom lie outside the ONOC plane. The extension of the Cordell, Boggs, and Skancke model suggests that it should lie in the ONOC plane. The other nitrite of a secondary alcohol studied here, 2-butyl nitrite, does not provide an unambiguous test of their model. The 2580-MHz species which we have assigned to an AGA conformation is also consistent with an ASA assignment predicted by their model. The only nitrite of a tertiary alcohol included in this study which yields information about 72 is tert-amyl nitrite. The assignment of the 2278-MHz species to an AAA conformer is quite firm and indicates that a C-CH:, bond does not have a strong stabilizing interaction when eclipsed with the nitrogen atom. Five of the ten alkyl nitrites studied here have three internal degrees of freedom of asymmetric rotors. The three nitrites of primary alcohols in this group, n-propyl, neohexyl, and isobutyl, show very similar conformational properties. Considering first those species syn about T ~ , n-propyl nitrite displays syn-anti-anti and syn-antigauche (or syn-gaucheanti) conformers. The single stable rl(ONOC) s y n conformer of isobutyl nitrite has ONOCCC configurations identical with the two preferred assignments of the n-propyl compound. Neohexyl nitrite displays only the syn-anti-anti configuration because of the steric bulk of the terminal tert-butyl group. Considering anti forms about T ~ all , three compounds have a single stable conformation with an anti-gauche-anti ONOCCC configuration. The branched-chain isobutyl nitrite simultaneously also contains an anti-gauche-gauche ONOCCC configuration. Finally, the LRMW spectra of n-propyl, neohexyl, and isobutyl nitrites are each dominated by a broad and intense band series from a species about 1kcal/mol higher in energy than each compound‘s most stable isomer. These
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
2335
are assigned to those species freely rotating about 7 2 (NOCC) above a low (-0.5 kcal/mol) internal rotation barrier. Stated differently, these bands are the ‘band heads” of the torsionally excited states of T~ The only nitrite with three asymmetric internal rotors and made from a secondary alcohol is 2-butyl nitrite. Of the 10 alkyl nitrites studied here, its LRMW spectrum is the most difficult to interpret. It is isopropyl nitrite with an added methyl group. The syn-gauche-anti, antigauche-anti, anti-gauche-gauche, and anti-gauchegauche’ conformers of 2-butyl nitrite are all reasonable and expected from our results of stable syn-gauche and antigauche conformers in isopropyl nitrite. The surprise is that there appear to be no bands due to species freely rotating about 72. Of the nine alkyl nitrites with asymmetric internal rotors about T~(NOCC), 2-butyl nitrite is the only one which does not have bands assigned to freely rotating species. Since LRMW band spectra only appear when a molecule is a nearly symmetric rotor, and since 2-butyl nitrite is about as asymmetrical as any of the compounds studied here, we assume the bands are too broad to be observed. The a-type R-branch bands of this compound are broader than those of any other in this study. tert-Amyl nitrite can be considered as tert-butyl nitrite with an added methyl group. Like tert-butyl nitrite it is observed only with 71(ONOC)anti. The anti-anti-anti and anti-gauche-anti stable species are consistent with the observed anti-tert-butyl conformer. The interesting feature is the presence of a high-energy species freely rotating about T~(NOCC). This suggests that there is probably also a low T~ barrier in tert-butyl nitrite. Its LRMW band spectra cannot demonstrate this since the tert-butyl group is axially symmetric. Finally, there are two compounds containing four asymmetric internal rotors, n-butyl and isopentyl nitrites. The conformational possibilities for these large molecules are so many, 34, that the assignments must be considered as reasonable but not definitive. Nevertheless, it is unambiguously clear from LRMW spectroscopy that n-butyl nitrite contains at least four spectroscopically distinguishable species. The most intense band series, like those of most of the other alkyl nitrites studied here, is the broadest and its variation of intensity with temperature is consistent with a higher energy (1.5 kcal/mol) than the other species. Its B + C value, 1773.8 (5) MHz, is consistent with a species with anti configurations about T ~ T, ~ and 74 and freely rotating about T~(NOCC).The 1789 (3) MHz species is assigned to the bound states about 72, AGAA configuration, and the 1838.5 (7) MHz species to the SAAA configuration. An AAAG species would have a very similar B + C value, but analogous configurations are not seen in simpler compounds and it is unlikely here. The LRMW spectra of isopentyl nitrite are similar to those of n-butyl nitrite. The two series of the most stable species, B C values of 1564 (1)and 1642 (1)MHz, are assigned to SAAG and AGAG (or AG’AG) configurations. They correspond to the SAAA and AGAA forms of n-butyl nitrite since the fourth torsional angle describes a CCCH configuration in isopentyl but a CCCC in n-butyl nitrite. The SAGA species observed in n-butyl nitrite does not have an observable counterpart in isopentyl nitrite probably because the added bulk of a methyl group destabilizes the gauche configuration about T ~ .
+
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. The authors are grateful to Professor Marlin D. Harmony of the University of Kansas and E. Bright Wilson of
,
2336
J. Php. Chem. 1982, 86,2336-2339
Harvard University for use of their microwave spectrometers. The Harvard spectrometer is supported by NSF grant GP-37066x, and the Kansas spectrometer by NSF grant MPS-74-22178. We are also grateful to Dr. Paul Turner for communicating his results before publication and for many stimulating discussions. We thank one of the referees for unusually perceptive, thorough, and constructive criticism. Calculations were carried out at the University of Connecticut computer center and at Lawrence Berkeley Laboratory. This research was supported in part by the National Resource for Computation in Chemistry under a grant from the National Science Foundation (Grant No. CHE-7721305) and by the Basic
Energy Science division of the US Department of Energy (Contract No. W-7405-ENG-48). Supplementary Material Available: Tables of observed band frequencies, J 1 values, and B C values for neopentyl, n-propyl, neohexyl, isobutyl, n-butyl, isopentyl, isopropyl, 2-butyl, tert-butyl, and tert-amyl nitrites. Also, figures of B + C vs. v for torsionally excited neopentyl nitrite, intensity of the AFr band series vs. Stark field for neopentyl nitrite, and calculated B + C values for isopropyl, n-propyl, neohexyl, isobutyl, 2-butyl, tert-amyl, n-butyl, and isopentyl nitrites (22 pages). Ordering information is given on any current masthead page.
+
+
Dephasing of Optical Phonons in a Substitutionally Disordered Organic Solid. The Lowest-Frequency Raman-Active Phonon of a Binary Solid Solution between p -6romochiorobenzene and p -Dichlorobenzene Larry A. Hess and Paras N. Piasad'+ Department of Chemistry, State Universlty of New York at Butfalo, Buffalo, New York 14214 (Received:June 9, 1981; I n Final Form: December 2, 198 1)
The temperature dependence of the line width is investigated for the lowest-frequency Raman-active phonon (28 cm-') in a 1:l molar solid solution of p-dichlorobenzene (DCB) and p-bromochlorobenzene (BCB). It is found that, at 2 K, the line width is dominated by the contribution due to substitutional disorder, yet the line shape is a Lorentzian. This result is in agreement with the prediction of a theory based on a configurationally averaged Green's function. The temperature-dependence study is successfully used to determine the mechanism of dephasing due to anharmonic phonon-phonon interactions. The anharmonic contribution of the line width in the solid solution fits the mechanism of dephasing due to a TI relaxation in which the optical phonon decays into two acoustic phonons of half its frequency by cubic anharmonic interactions. The same mechanism has been found to explain the temperature dependence of line widths of the corresponding phonons in the p-dichlorobenzene and p-bromochlorobenzene neat crystals.
Introduction Dephasing of an optical phonon can be studied by either a coherent transient technique (time-domain measurements such as time-resolved CARS) or a study of the line width (frequency-domain measurements) of the optical tran~ition.'-~The T2relaxation, responsible for dephasing, has been found3" to be in subnanoseconds for low-frequency optical phonons in organic crystals at
