334
J . Phys. Chem. 1984, 88, 334-341
C-H Stretching Modes and the Structure of n-Alkyl Chains. 2. Long, All-Trans Chains R. A. MacPhail,+H. L. Straws, R. G . Snyder,* Department of Chemistry, University of California, Berkeley, California 94720
and C . A. Elliger Western Regional Research Center, US.Department of Agriculture, Berkeley, California 9471 0 (Received: April 28, 1983)
The C-H and C-D stretching bands in the infrared spectra of some crystalline n-alkanes at low temperature (99%) and have been described in the literature.8 Shorter rz-alkanes were obtained from commercial sources and were a Estimated averages from Table 11. Calculated values. See typically of spectroquality or better (>99% purity). NonadeFR stands for Fermi resonance component. Table V. cane-d40 was obtained from Merck and Co. (Canada) with 98 atom % D. Urea clathrates of the n-alkanes were crystallized from attribute to the compound in which one deuterium was replaced methanol/2-propanol mixtures by using standard procedure^.^ by a hydrogen. From the relative intensities of these peaks we The samples of n-CD3(CH2)20CD3and I I - C H ~ D ( C H ~ ) ~ & H ~ D , which were prepared by standard method^,^ were shown by mass (1) For example, see: Wallach, D. F.; Verma, S. P.; Fookson, J . Biochim. spectroscopy to be of high purity with respect to chain length Biophys. Acta 1978, 559, 153-208 for Raman application. Casal, H. L.; Cameron, D. G.; Jarrell, H. C.; Smith, I. C. P.; Mantsch, H. H. Chem. Phys. (>95%). However, the mass spectrum of each of these samples Lipids 1982, 30, 17-26 for infrared application. showed a peak one m / e unit below the parent peak, which we 'Current address: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024.
0022-365418412088-0334$01.50/0
(2) Snyder, R. G.; Straws, H. L.; Ellinger, C. A. J . Phys. Chem. 1982, 86, 5145-50. ( 3 ) MacPhail, R. A.; Snyder, R. G.; Straws, H. L. J . Chem. Phys. 1982, 77, 1118-37.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 335
C-H Stretching Modes and n-Alkyl Chains
d+
W
al
C
n
C o n
n
n
i
i
a
a
3000
'
2950
'
2900 Wavenumbers
28'50
'
28'00
Figure 1. C-H stretching region of the infrared spectrum of n-CD3(CH2)&D3 at 7 K. The sample was in a pressed KBr pellet. (See Table I for assignment notation.)
calculate the percentage of chains with one less methyl deuterium to be 17% for n-CD3(CH2)20CD3and 11% for n-CH2D(CHJz0CH2D. Since there are two methyl groups per chain, the number of methyls with the desired degree of deuteration is approximately 91% for the former sample and 94% for the latter. Infrared spectra of the n-alkanes and n-alkane-urea clathrates, which are crystalline at room temperature, were measured with the samples in pressed KBr pellets or, in the case of the n-alkanes, as films crystallized from the melt onto CsI plates. Infrared spectra were measured by using either a dry-air-purged Nicolet 7 199 FTIR or a Nicolet 8000 HV FTIR with an evacuated optical path and nitrogen-purged sample chamber. Both TGS and liquid-nitrogen-cooled MCT/InSb detectors were used. The optical retardation length was chosen to give 0.5-cm-' resolution, and with the use of Happ-Ganzel apodization the effective resolution was about 0.7 cm-'. Samples were cooled in either a Displex Model CS-202A or a Lakeshore Cryotonics CTI Model 21 closed-cycle helium refrigerator, and temperatures were monitored at the cold station with the use of an Au-Fe vs. chrome1 thermocouple, or a silicon diode, respectively. The spectrometer and the cooling apparatus used in the Raman experiments have been described elsewhere.I0
Wavenumbers
Figure 2. C-H stretching region of the infrared spectrum of n-C20H42 at 9 K. The sample was in a pressed KBr pellet.
1
1 d+
Wavenumbers
Figure 3. C-H stretching region of the infrared spectrum of n-CZIH44 at 7 K. The sample was in a pressed KBr pellet.
111. Designation of Modes
Our notation, which is similar to that of Snyder et al.,7 is summarized in Table I. Methylene C-H (C-D) stretches are designated by d, with d+ and d- referring respectively to the symmetric and antisymmetric infrared-active fundamentals of the polymethylene chain with a planar carbon skeleton. The subscript w refers to the terminal methylene group. Methyl C-H (or C-D) stretches are designated by r. More specifically, r+, r a , and r-b denote the symmetric, in-plane asymmetric, and out-of-plane asymmetric modes where in-plane and out-of-plane refer to the plane of the carbon skeleton of the n-alkane. In the case of the partially deuterated methyl groups CH2D and CHD2, the subscripts ip (in-plane) and op (out-of-plane) refer to the H or D (4) Hill, I. R.; Levin, I. W. J . Chem. Phys. 1979, 70, 842-51. (5) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acta 1980, 601, 47-53. (6) Snyder, R. G.; Scherer, J. R. J . Chem. Phys. 1979, 71, 3221-8. (7) Snyder, R. G.; Hsu, S. L.; Krimm, S . Spectrochim. Acta, Part A 1978, 34, 395-406. Smith, ; A. E.; Skinner, L. B. J . A m . (8) Schaerer, A. A.; Busso, C. .I. Chem. SOC.1955, 77, 2017-9. (9) For example, see: McAddie, H. G. Can. J . Chem. 1962,40,2195-203. (10) Scherer, J. R.; Snyder, R. G. J . Chem. Phys. 1980, 72, 5798-808.
W C
-? YI
n
a
J
3000
2950
2900 Wavenum bers
2850
2800
Figure 4. C-H stretching region of the infrared spectrum of the nC,,H,,-urea clathrate a t 9 K. The sample was in a pressed KBr pellet.
primarily involved in the vibration and, as before, the plane refers to that of the carbon skeleton.
336 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
MacPhail et al.
TABLE 11: Summary of C-H and C-D Stretching Mode Assignments for Methylene and Methyl Groupsa ___---
d-
CH,(chuin)
CH D
2890 2894 2890 2890 2890 d, 29G (2892)d
r -a
r-b
2962 2962 2960 2964 295 8 2954
295 1 2953 2950
2845 2847 2846 2845 2849
~+FR
r+ 2869
crystal f o r m b
C2nH42
T
C21H44
0
CD,(CHz)znCD, CH,D(CH,),,CH,
?
?
D
uc
1 CHD in c,,D,, 2930 293 1
2946
2870 28621
1
0
C20H42
T
C?,1H44
0
CH, in CH,D(CH,),,CH,D
?
1C,,H,,
2867
uc
r-oP.oP
2955
2947
lip
CHD,
n-alkane
dt
2853 2853 2854 2852 2854
1
r-ip,op
CH, D
d',
~ + F R
2914 2915 291 5 291 5 2922 d 2892' 2881'
TOP
2940
2927c 2925c 2928
2939 r -a -b 2224 221 9 2210
~+FR
I+
2123 21 1 8
2073
Frequcncies ( c n i r ' ) are for the sarnples a t T < 1 0 K. T = triclinic, 0 = orthorhoinbic, UC = urea clathrate. Estimated frequency coincides with d(CHD) band at 2892 ciii-'.
Site-split components.
a
TABLE 111: CH, and CH, Stretching Frequencies: Relative Intensities,b and Assignments'
2990 br, w 2964 w. sh 2958 ni 2954 yh,
2962 I"
111
1
111 2953 1111 2946 m 29.50 2930 i h , 111 2931 111, 511
2951
1-b
-
2920
2600
2880 I
2915
29145
\
2915
2854 111 2846 s
111
2890 br, 2883 w 2870 ni 2867 111 2953 in, c h 2854 sh. 111 28475 28495
2890 br, m 2894 br, 2869 in 2853 sh. 111 2845 5
111
Peak positions a r c listed to the nearest c n - ' . mediuni, w = weak, sh = shouldcr. br = broad. and Table I for notation. a
111 =
2840 I
I
F
2902 w 2890 br.
2860
cm-I
7
1
d+FR I+
d', d'
= strong;
' See text
Some fundamentals have more than one component due to Fermi resonance interaction and, in that case, the component that we deem to consist of the largest fraction of the fundamental vibration is designated as the fundamental. To designate the complementary components, the subscript FR is used.
IV. CHz and CH3 C-H Stretching Modes Our assignments for the C-H stretching vibrations of C H 2 methylene and CH, methyl groups, which are essentially the same as those of Snyder et al.,' are listed in Table 111. These assignments are based on the infrared spectra of n-CD3(CH2)20CD3, n-C20H42,n-C21H44,and the n-C16H34-ureaclathrate which are shown in Figures 1-4. Neat n-Alkanes. First we consider the methylene C-H stretching fundamentals. These can be unambiguously identified from the spectrum of n-CD3(CH2),oCD3(Figure 1). The strong, sharp, and somewhat asymmetric bands near 29 15 and 2846 cm-' belong to the d- and d+ modes. The less intense, high-frequency shoulder on the d+ band near 2854 cm-I is assigned to the d+ mode associated with the methylene group next to the methyl group as first suggested by Hill and L e v h 4 In support of this assignment,
cm-l Figure 5. Calculated infrared spectrum of the d+ fundamental perturbed by Fermi resonance interaction with binary combinations of the HCH bending modes. The calculation was based on the treatment given for the Raman case in ref 6 with K i l l = 56 cm-I. The results are for a chain with 18 methylenes. The region near 2900 cm-' is shown in the lower
spectrum with an expanded intensity scale. we note that the intensity of the band at 2854 cm-' relative to that of the band at 2846 cm-' increases with decreasing chain length. A less intense, broad band near 2890 cm-' is attributable to Fermi resonance between d+ and the overtones of the methylene bending (scissors) modes. Snyder and Scherer6 have analyzed in detail this same kind of resonance in the Raman spectrum. In the Raman case there is a broad band near 2900 cm-I in addition to sharp bands at 2850 and 2930 cm-'. These bands arise because the overtones of the modes along the H C H bending dispersion curve all have the proper symmetry to interact with the zone-center ( k = 0) d+ f ~ n d a m e n t a l .In ~ the infrared case, the interaction is not with overtones of the HCH bending modes but with binary
C-H Stretching Modes and n-Alkyl Chains combinations whose components have phase angles 4 and T-4, respectively. We have analyzed this interaction in the same manner that we used for the Ramam6 The calculated d+ spectrum (Figure 5) shows a strong band near 2845 cm-', a weaker band near 2915 cm-', and a series of weak, closely spaced bands, which, if unresolved, will produce a broad band centered at about 2895 cm-'. This is essentially the pattern that we observe. We note that the weak band calculated at 2915 cm-' is obscured by the intense d- band. Infrared polarization measurements on n-alkane single crystals indicate that the bands at 2898 and 2920 cm-' are indeed components of d+." Although Snyder and Scherer were able to resolve the individual components of the broad band in the low-temperature Raman spectrum of the n-C,6H34-urea clathrate, we have not been able to do so for the analogous components in the infrared, probably for the following reasons. For chains of the length that we have studied here, the component separation in the infrared is calculated to be only about 3 cm-', approximately half that for the Raman components and comparable to the width of the narrowest lines observed in our low-temperature infrared spectra. Moreover, in the case of the crystalline n-alkanes the individual components ma:y be broadened by intermolecular i n t e r a ~ t i o n . ~ , ~ Etands associated with the CH, methyl group are revealed by comparing the C-H stretching spectra of n-C20H42and n-CD3(CH2)20CD3.The n-alkane n-C20H42has a particularly simple CH stretching spectrum (Figure 2) since this molecule exists in a triclinic crystal structure with only one molecule per unit ce11.12 The absence of factor-group splitting for the 720-cm-' methylene roclung mode confirms that our sample has this crystal form. The spectrum of ~ I - C ~ ~ H (Figure ,, 2) has bands at 2962, 2951, 2930, and 2869 cm-I which do not appear in the spectrum of n-CD,(CH2)20CD3(Figure 1) and which therefore must belong to the CH3 group. Earlier polarization measurements on n-alkane single crystals" have shown that the 2962- and 2951-cm-' bands are the r-, and rmbasymmetric stretching modes. These modes would be degenerate if the methyl group had C, symmetry but attachment to the n-alkane chain lifts the degeneracy: the in-plane and out-of-plane CH bonds of the methyl are no longer equivalent. The: band at 2869 cm-' and the shoulder near 2930 cm-I are assigned to components arising from Fermi resonance between the symmetric methyl stretch r+ fundamental and the overtones of the methyl asymmetric bending modes, which appear near 1450 cm-1.13913A detailed discussion of this resonance appears in section VII. The C-H stretching spectrum of n-C2,H4, (Figure 3) is similar to that of n-C20H42except that the r-a and r-b bands each appear as doublets whose components are separated by about 3 cm-'. The doubling is the result of intermolecular vibrational coupling (factor-group splitting) which occurs for odd n-alkanes in the orthorhombic structure described in ref 14. The splitting of the 720-cm-' methylene rocking band shows our sample to be either orthorhombic or monoclinic while the splitting and intensity pattern of the methylene rocking-progression bands between 720 and 900 cm-' rules out the monoclinic form.I5 Therefore, we attribute the doubling of the r- modes to factor-group splitting for ithe orthorhombic form. The observed 3-cm-' splitting of the components is near that calculated for the methylene stretches of orthorhombic polyethylene.I6 It is interesting that the r+ component at 2870 cm-' does not show this splitting. n-Alkaneurea Clathrates. The ~ z - C ' ~ H ~ ~ - uclathrate rea CH stretching spectrum appears in Figure 4. In the clathrate crystal, the alkane chains are held in their fully extended all-trans conformation and are stacked end-to-end in channels formed by the ureal lattice." The environment of an n-alkane in a urea clathrate is thus quite different from that in the neat crystal. Nevertheless, (1 1) Holland, H. F.; Nielsen, J. R. J . Mol. Spectrosc. 1962, 8, 383-405. (1 2) Muller, A,; Lonsdale, K. Acta Crystallogr. 1948, 1 , 129-3 1. (13) Snyder, R. G. J . Mol. Spectrosc. 1960, 4 , 411-34; 1964, 7, 116-44. (14) Smith, A. E. J . Chem. Phys. 1953, 21, 2229-31. (15) Snyder, R. G. J . Chem. Phys. 1979, 71, 3229-35. (16) Tasumi, M.; Shimanouchi, T. J . Chem. Phys. 1965, 43, 1245-58. (17) Smith, A. E. Acta Crystallogr. 1952, 5, 224-35.
The Journal of Physical Chemistry. Vol. 88, No. 3, 1984 337
Wavenumbers
Figure 6. C-H stretching region of the infrared spectrum of the nCloH22-urea clathrate a t 9 K. The sample was in a pressed KBr pellet.
the C-H stretching spectra of the clathrate (Figure 4) and of a crystalline n-alkane (Figure 3) are very similar. Differences can be seen in the shape of the d- band near 2920 cm-' and in the detail of the d+ Fermi resonance bands in the region near 2890 cm-'. The d+ Fermi resonance is known to be sensitive to the intermolecular environment5-' so the changes in the d+ band are not surprising. The r- bands in the clathrate-crystal spectra are broader, and some of them have a discernible structure that may be due to interactions between methyl groups on adjacent chains. Several weak, broad features that are not observed in the spectra of the neat alkanes can be seen in Figure 4. These may be attributable to the urea host or to alkane vibrations that are forbidden in the spectrum of the neat crystal but allowed in the clathrate. Finally, the C-H stretching frequencies of the clathrate are slightly different from those of the neat alkane. In going from the clathrate to the neat n-alkane, the frequencies of the methyl bands increase while those of the methylene bands decrease. For comparison we have also included a spectrum of the C-H stretching region of the n-CloH22-urea clathrate (Figure 6). Several weak features appear in this spectrum that are apparently due to neat hydrocarbon crystals.'* The low-frequency shoulder on the side of the d+ band near 2850 cm-' and the high-frequency shoulder on the side of the r+ band near 2870 cm-' correlate well with the neat alkane frequencies listed in Table 111. However, this spectrum is useful in corroborating our assignments since it clearly shows that with decreasing chain length the intensity of the methyl and terminal methylene bands increases relative to the intensity of the bands from the nonterminal methylenes. We note that the two intense r- bands near 2950 cm-I are narrower and are separated (-8 cm-') less than the corresponding bands for the n-CI6H3,clathrate. A detailed discussion of the widths and separations of the r- components can be found in ref 3. V. C-H Stretching Modes of CHD, CH,D, and CHD, Groups In this section we discuss assignments of partially deuterated methylene and methyl groups based on the spectra of samples of perdeuterated n-nonadecane and docosane- 1,22-d2. Perdeuterated n-Nonadecane Sample Containing Residual Hydrogen. The C-H stretching region of the spectrum of a sample of n-C19Da containing about 2% hydrogen as an isotopic impurity is shown in Figure 7, and observed frequencies and assignments are listed in Table IV. The most intense bands are due to CHD and CHDt groups adjacent to fully deuterated groups. Most of (18) Several other n-alkane-urea clathrates were studied which also showed significant amounts of neat n-alkane impurities. Since these samples were run as KBr pellets, the impurities may have been introduced during the grinding or pressing procedures.
338 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
MacPhail et al. TABLE V: Calculated' C-H and C-D Stretching Frequencies (cm-') of Isotopically Substituted Methyl Groups
d (CHD)
C-D stretching
C-H stretching modeb r a
;
group
freq
group
CH,
2959 2945 2863 2956 2897 2945 2890 2938 2918
CD,
221 1
CHD,
2200 2065 2210
Tb
0 W
'-ip,op
0
riip,op
e
CH,D
OP,OP
s: n
'+OP,OP
a
CHD,
rip 'OP
'Calculated
freq
2112
CH,D
2202 2105 2168 21 53
t o r trans-n-pentanchaving deuterated mcthylene See text and Table I for notation.
groups. See tcxt.
3 Wavenumbers
Figure 7. C-H stretching region of the infrared spectrum of n-CI9D,, (hydrogen impurities) at 7 K. The sample was crystallized from the melt onto a CsI plate. The bands designated by an asterisk probably belong
to the undeuterated n-alkane (see text).
W 0 C
TABLE IV: C-H Stretching Frequencies and Assignments of Hydrogen Impurity Bands in n-C,,D,, peak position' intensity assignmentC 2961 W r2953 W r -:d 2944 VW 2940 in 2921 2925 111 In 2923 sh, m 2915 W 2903 m d,(CHD)e 2899 sh, w 2892 F d(CH:) 2888 sh, vw ~ + F R 2881 S d(CHD) 2872 sh, w r+ 2856 2850 111 vw ;:y 2848 sh, w
t
'
Peak positions are listed to the nearest c n - ' . s = strong, See in = medium, w = weak, vw = v e r y weak, sh = shoulder. test and Table I for notation. From a n-C,,H,,, impurity. Associated with thc 2892-cill~'component.
the remaining bands appear to be accountable to a small amount of the undeuterated n-alkane. The C-H stretches associated with the CHD methylene group are manifest in the intense bands at 2892 and 2881 cm-I, which have been previously observed in the spectra of perdeuterated he~atriacontane~ and CD3CHDCD3.I9 The appearance of two bands instead of one is attributed7,l9to site-group splitting since in the crystal the skeletal plane of the n-alkane is no longer a plane of symmetry, and consequently the two methylene hydrogens are no longer equivalent. The C-H stretch frequency of a C H D methylene adjacent to a methyl group is slightly higher than that of other methylenes in the chain4 Our unpublished gas-phase Raman measurements indicate that the frequency of a deuterium isolated C-H stretching mode of a terminal methylene (CHD) is about 14 cm-' higher than that of a nonterminal methylene. This makes it probable that the 2903-cm-' band belongs to the d,(CHD) associated with the higher frequency component of d(CHD). The estimated position of the lower frequency com(19) McKean, D. C.; Biedermann, S.; Biirger, H. Spectrochim. Acta, Part A 1974, 30, 845-57.
;
n
a
3 10
2950
2900
2850
2800
Wovenumbers
Figure 8. C-H stretching region of the infrared spectrum of n-CDH2(CH2)20CDH2 at 7 K. The sample was in a pressed KBr pellet.
ponent of d,(CHD) is 2892 cm-I, and therefore this component will be buried under the 2892-cm-' d(CHD) band. To aid in the assignment of the methyl group stretching vibrations, we have calculated the vibrational frequencies and normal coordinates of trans-n-pentane with variously deuterated methyl groups. The force field used is that of ref 20, except that the diagonal force constants for the in-plane and out-of-plane methyl C H stretching coordinates were allowed to differ slightly so as to reproduce the observed splitting of the r-, and r-bmodes. The results of this calculation are presented in Table V. A comparison of the calculated frequencies with those observed for the n-C19D40sample (Table IV) allows us to assign the band at 2940 cm-l and the split band centered at 2926 cm-' to the in-plane and out-of-plane CHD2stretching vibrations, respectively. The out-of-plane mode is apparently site split into components at 2927 and 2925 cm-' in the same manner as the methylene CHD stretches. This assignment has also been proposed for the lowtemperature C H stretching spectrum of CD3CD2CHD219where the out-of-plane C-H band is again found to be split although by a somewhat larger amount. The weak bands at 2961,2953, 2923, 2888, 2872,2856, and 2850 cm-' correspond in frequency to an undeuterated n-alkane, and in Table IV we have assigned them accordingly. The presence of a small amount of the undeuterated n-alkane was unambiguously identified in a sample of perdeuterated tetracosane also obtained from Merck and Co., and this makes it likely that the same type of impurity is present in our perdeuterated n-nonadecane sample as well. Several very weak bands remain unassigned. Among the possible origins of the unassigned bands are modes from the (20) Snyder, R. G. J . Chew. Phys. 1967, 47, 1316-60.
The Journal of Physical Chemistry, Vol. 88. No. 3, 1984 339
C-H Stretching Modes and n-Alkyl Chains
peak positiona 2964 2955 2947 2939 2928 2915 2890 2862 2852 2845
intensityb
A. KBr Pellet
r
TABLE VI: C-H Stretching Frequencies and Assignments for n-CH, D(CH ,),,CH, D assignmentC
w in
in
sh, w ni
s
br.
511, in
sh, w ah, w s
a Peak positions arc listed to thc nearest cin-'. See Table 111 for notation on intensities. See Tablc I and text for notation.
2250
TABLE VU: CD, Stretching Frequencies and Assignments for n-CD,(CH,),,CD, peak position: sample 1
sample 2d
sample 3e
2225 sh, m 2219s 2208 s 2203 sh
2224 m 2219 s 2210 F 2203 sh 2201 sh 2199 i n 2194sh 2193 m 2132 w
2195br,ni
2194br,m
2131 s h , w 2121 111
2133 w 2121 in
2078 sh, 2073 s
2079 sh, ni 2073 s
in
22118 123m 2079 w 2073 s
2150 Wovenumbers
1 'A 1
assinnmcnt
2100
2050
Crvstollized From Melt 8. Crystollized
r
intensityb
2224 sh, m 221 8 s 2208 s 2203 sh
2200
(or thor hom bic)
I
resonances with DCD bending combinations u C 0
.
P
n
I
r+FR r+
'
a Peak positions are listed to the nearest c m - ' . See Tables I and 111 for notation. Sample 1 was found to have mixed triclinic and orthorhombic crystal forms. Sample 2 had an orthorhombic crystal structure. e Sample 3 was diluted (approximately 1 : l o ) in n-C22H46.
following: (i) adjacent CHD groups (these are calculated to occur in the region between 2900 and 2870 cm-'), (ii) isolated CH2D and CH2groups, and (iii) 13C-H stretching (such modes will have frequencies -9 cm-' lower than their 12C-H counterparts). Docosane-1,22-d2. The C-H stretching modes of CH2D groups are seen in the spectrum of n-CH2D(CH2)20CH2D shown in Figure 8, and the observed peak positions and assignments for this molecule are listed in Table VI. The medium strength bands at 2955 and 2947 cm-l must be the in-plane, out-of-plane (ip, op) and the out-of-plane, out-of-plane (op, op) asymmetric CH2D stretches. We noted in the section on Experimental Procedures that approximately 10% of the methyls in the n-CH2D(CH2)20CH2Dsample are CH3 groups so that the weak band at 2964 cm-' is probably the r-a mode for this group; the r-b band would then lie under the CH2D band at 2955 cm-'. This would account for the apparent reversal in the relative intensities of the antisymmetric CH,D stretching modes that occurs in going from n-CH2D(CH2)20CH2Dto n-C19D40. Finally, we note that the bands at 2939 and 2928 cm-' may be assigned to the r,,, and rop modes of the CHD, group.
VI. C-D Stretching Modes of CD3 and CHzD We now discuss the assignments of the C-D stretching modes of the methyl groups in the two molecules n-CD3(CH2)20CD3 and n-CH2D(CH2)20CH2D.Infrared spectra of these samples, which have been measured both neat and diluted in n-C2,Hd6,appear in Figures 9 and 10, and the observed CD, bands and their assignments are listed in Table VII. Docosane-I ,I ,I ,22,22,22-d6. Figure 9 shows spectra of CD3(CH2)20CD3samples prepared in three different ways. Figure 9A is for the sample in a pressed KBr pellet. Factor-group components as well as unsplit components can be identified in the
1
2250
' '
2200
2150 21'50
2100 21'00
'
2050 2650
Wavenumbers
1
C. In n-C22H46 I I : l O ) Crystallized From Melt
Wovenumbers
Figure 9. C-D stretching region of the infrared spectrum of n-CD,(CH2)20CD3at 7 K: (A) The sample was in a pressed KBr pellet and showed mixed triclinic/orthorhombic crystal forms. (B) The sample was crystallized from the melt onto a CsI plate and showed an orthorhombic crystal structure. (C) The sample was diluted in n-C22H46(1:lO) and was crystallized from the melt onto a CsI plate.
methylene bending (1470 cm-I) and rocking (720 crn-l) regions and indicate that the sample was a mixture of at least two crystal forms (in roughly equal concentrations), probably triclinic and orthorhombic. The stable crystal form for pure n-C22H46is triclinic, but impurities or certain conditions of crystallization may cause the n-alkane to become orthorhombic.21 A second sample of ~ Z - C D ~ ( C H ~ )crystallized ~ ~ C D ~ , from the melt onto a CsI plate, (21) Broadhurst, M. G. J . Res. Natl. Bur. Stand., Part A 1962, 66, 241-9.
The Journal of Physical Chemistry. Vol. 88, No. 3, 1984
340
A . KBr Pellet
0)
c 0
.
2 ln
n
a -
4.1
2250
2300
1
23-00
2200 Wavenumbers
B. In n-C22H46 (1:iO) Crystallized From Melt
-
2i50
2200
2150
2100
n
'
21'50
'
2100
Wavenumbers
Figure 10. C-D stretching region of the infrared spectrum of nCDH2(CH2)20CDH at 7 K: (A) The sample was in a pressed KBr pellet. (B) The sample was diluted (-1:lO) in n-C22H46 and was crystallized from the melt onto a CsI plate.
had an orthorhombic crystal form, as evidenced by the factorgroup split rocking mode at 785 cm-'.15 Its C-D stretching spectrum appears in Figure 9B. While the spectra in Figure 9, A and B, are similar, there are differences in the bandshapes, and there are some additional features in Figure 9B. To further resolve the underlying features and eliminate the complications of factor-group splittings, a third sample was crystallized onto a CsI plate from a melt of ~ Z - C D ~ ( C H ~diluted ) ~ ~ C at D least ~ 1:lO in n-Cz2Hd6. The spectrum of this sample, shown in Figure 9c, is significantly different from the spectra of Figure 9, A and B. The frequencies and relative intensities of the observed bands for these three samples are listed in Table VI1 along withour assignments. We have not included the many weak bands in this region, most of which are probably attributable to binary combinations from the twisting-rocking progressions.22 The bands at 2073 and 2121 cm-' are assigned to components resulting from Fermi resonance interaction between r+ and the methyl asymmetric bending overtones. A similar assignment has been made by others for CD3 groups in n-alkylcarboxylic a ~ i d sand ~ , pro~~ ~ a n e . ' ~However, .~~ there are additional complications, and these (22) Snyder, R. G . J . Chem. Phys. 1978, 68, 4156-66. (23) Sunder, S.; Mendelsohn, R.; Bernstein, H. J. Chem. Phys. Lipids 1976, 17, 456-65. (24) Lavalley, J. C.; Sheppard, N. Spectrochim. Acta, Parr A 1972, 28, 2091-101.
MacPhail et al. also appear to arise from Fermi resonance interactions. For example, in the isotopically dilute sample (Figure 9C) the band at 2121 cm-' becomes a doublet, and the weak band at 2079 cm-I, which appears as a shoulder on the intense 2073-cm-' band for the neat n-alkane (Figure 9, A and B), is resolved. Our calculations (Table V) indicate that the r-a and r b bands should appear near 2210 cm-* and that their frequency separation should be 1/2lI2 times the separation observed for the CH3 group; Le., the separation should be 6-9 cm-'. However, our spectra (Figure 9) show more than just two components. Even in the spectrum of the isotopically diluted sample there are three sharp bands at 2224, 2217, and 2209 cm-' together with a broad cluster of weaker bands near 2200 cm-I, and there is no obvious way to associate these with the r-a and r-b modes. It is important to note that the complicated structure in this region persists in the dilute spectrum. Therefore, it is likely that most of the complexities arise from intramolecular perturbations, although some features in these spectra could be accounted for by CHD2 groups whose concentration is about 10%. In section VI1 wetdiscuss possible resonances involving r+ and r- C-D stretching modes. Docosane-1 ,22-d2. Spectra in the C-D stretching region of T I - C H ~ D ( C H ~ ) ~ ~ are C H shown ~ D in Figure 10A for the neat alkane and in Figure 10B for the n-alkane diluted in n-C22H46. Both samples were crystallized from the melt on a CsI window. In the case of the diluted sample, the mole fraction concentration of n-CH2D(CH2),,CH2D in n-C22H46was 0.1 or slightly greater. Further dilution proved impractical due to the presence of combination bands in this region from the In both spectra, the methylene bending (1470 cm-I) and rocking (720 cm-') bands are not split, indicating that the samples were in the triclinic crystal form. The C-D stretching spectrum of this compound, as in the case of CD3(CH2)20CD3,is generally much more complex than the C-H stretching spectra that we have observed, thus frustrating a detailed assignment. Table V indicates that there should be two bands near 2160 cm-' that correspond to the in-plane and out-of-plane C-D stretches. From the observed separation of the analogous C-H stretching bands, we would estimate a splitting of about 10 cm-' for the C-D stretches. Instead, we see a single band at 2177 cm-' in both spectra in Figure 10, with a high-frequency shoulder near 2195 cm-l and a broader low-frequency shoulder near 2150 cm-'. The fwhm of the strong band is about 17 cm-' in the neat sample and 15 cm-' in the diluted sample. The C-H stretching bands of the methyl group show a characteristic temperature dependence3whereas the band at 2177 cm-' is essentially temperature independent. Although the 2177-cm-' band becomes slightly more symmetric in going from 7 to 300 K, it still maintains essentially the same width. We have measured Raman spectra of liquid n-CH2D(CH2)&H2D at 250 and 300 K and find that the isotropic spectrum is nearly identical with the infrared spectrum in Figure 10A: an intense band is observed at 2172 cm-' which has a broad shoulder near 2150 cm-' and which has a temperature-independent fwhm of 23 cm-'. These observations seem to imply, as in the case of the CD3 r- modes, that the C-D stretches are perturbed by intramolecular interaction, probably through resonances with combinations and overtones of bending modes.
VII. Resonances Involving C-H and C-D Stretching Modes of the Methyl Group In this section we discuss the resonances involving methyl group modes, resonances that seem to be ubiquitous in the C-H and especially the C-D stretching regions. First we address the Fermi resonance that splits the r+ mode and consider the character of the two components. Then we discuss certain bending modes whose combinations and overtones can interact with the stretches and thereby give rise to the complexities in the spectra of CD3 and CDHz described above. Symmetric Stretching Mode. The symmetric C-H (or C-D) stretching mode r+ of the methyl group CH, (or CD,) has two main components that appear near 2930 and 2870 cm-I for CH3
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 341
C-H Stretching Modes and n-Alkyl Chains and near 2121 and 2073 cm-' for CD, in both the infrared and Raman. In both kinds of spectra, the lower frequency component is the more intense. Therefore, in the assignment of this doublet to the Fermi resonance between the r+ mode and an overtone of one of the asymmetric bending modes of the methyl group,2 it is natural to assume that the lower frequency component contains more of the fundamental than does the higher frequency component. However, Hill and Levin4 have argued in favor of the opposite assignment and have used a "negative" Fermi resonance perturbation to explain why the intensity ratio of the components is the reverse of that expected. It is important to understand the r+ fundamental since it is used extensively as a diagnostic of structure. In arguing for their assignment, Hill and Levin cite the work of Dickson, Mills, and C r a ~ f o r on d ~deuterated ~ methyl halides in which an analogous resonance was observed and the highfrequency component was assigned to the fundamental. In this case, however, the unperturbed C-D stretching frequencies are higher than twice the asymmetric bending mode frequency, and so it is understandable that the high-frequency band is the more intense. The situation however, is different for the n-alkanes in that the unperturbed C-H and C-D stretching frequencies are lower than the binary frequency. In any event, the main problem with the Hill and Levin assignment lies in the necessity of assuming that the intensity of the unperturbed overtone of the bending mode is a significant fraction, nearly one-half, of the intensity of the unperturbed fundamental. The intensities of overtone and combination bands observed in the infrared and Raman spectra of these systems are very weak, and, in fact, in the case of the methylene group, Fermi resonance band structure in the Raman can be explained quantitatively on the assumption that the overtones of the bending modes have zero intensity.6 Therefore, a more satisfactory assignment is one in which the more intense r+ component contains the larger proportion of the fundamental. Asymmetric Stretching Modes. The symmetry requirement for resonance interaction with the r-', and r-b stretches of a CH, or CD, group is satisfied by a combination of the asymmetric bending vibration (a, or ab)with the symmetric methyl bend (U). In addition, because of the broken threeforld symmetry, the overtones 2aa and 2ab can interact with r-,, and the a, a,, combination can interact with rwb. However, for the CH3 group, none of these combination or overtone frequencies are high enough to interact significantly with the r-, and r-b bands near 2950 cm-' since the bending motions, a, and ab, occur at about 1450 and 1465 cm-' and U at about 1375 cm-1.11J3,20 In the case of CD3 groups, the situation is more favorable for resonance interaction with the C-D stretches. Our normal-coordinate calculations on CD3(CH2)$D3 indicate that a, is near 1050 cm-' and f f b is near 1040 cm-', while a mode whose potential
+
energy is about half U and half CC stretch is calculated at 1155 cm-'. We note that, because of mixing of U and CC stretching coordinates, there will be more than one vibration associated with U, and the frequency of these vibrations will be chain-length dependent especially for short chains. Lavalley and S h e ~ p a r d ~ ~ observed these modes in CD3CH2CD3at about 1060 and 1075 cm-' (a, and ab), and at 1120 cm-' (u + CC stretch), and in our infrared spectra we find a number of bands in this region. The cy modes are probably associated with two relatively strong bands near 1045 and 1055 cm-' and perhaps also with a somewhat weaker band at 1065 cm-'. There is another intense feature near 1120 cm-' which may be the U + CC stretch mode. In addition there are less intense bands near 11 10 and 1145 cm-I. The U a combinations of CD3 occur near 2200 cm-' in close proximity to the r- modes. In addition, if the U mode is assigned to the 1120-cm-I band, its overtone will be close to r-, and can interact with it because of the lowered symmetry of the methyl group. McKean et al.19 used this reasoning to explain the band at 2260 cm-' observed in the isotropic Raman spectrum of CD3CH2CD3.24 For the CH2D group the situation is similar. Our calculations indicate that C D bending and C-C stretching coordinates mix to give modes in the region 1150-1030 cm-'. The combinations and overtones of these modes fall in the region near the 2177-cm-' C-D stretching fundamentals and may account for the complex contours observed.
+
VIII. Summary We have measured the C-H and C-D infrared spectra of a number of n-alkane systems at low temperature and have assigned many of the bands. Fermi resonance has been observed and explained for the methylene symmetric C-H stretching fundamental of the chain. Particular attention has been paid to methyl group bands which show splittings due to the differences between the in- and out-of-plane C-H stretching force constants. These differences are clearly in evidence in the isolated C-H stretching bands that arise from the hydrogen impurities in a sample of n-Cl9D0 In analyzing the C-D stretching vibrations of n-alkanes with specifically deuterated methyl groups, we have found that the band pattern expected for nonequivalent methyl C-D oscillators is disrupted by resonances with combinations of methyl bending modes, which themselves are mixed extensively with skeletal modes. In general, complications due to Fermi resonance are much more conspicuous in the C-D stretching region than in the C-H region. Acknowledgment. We gratefully acknowledge support by the National Institutes of Health. We thank Dr. L. C. Leitch of the National Research Council of Canada, Ottawa, for the synthesis of some of the deuterium-substituted n-alkanes. Registry No. C2oH42, 112-95-8; C21H44, 629-94-7; CD3(CH2),,CD3,
(25) Dickson, A. D.; Mills, I. M.; Crawford, B., Jr. J . Chem. Phys. 1957, 27, 445-55.
83528-58-9;CHZD(CH2)2oCH2D,88130-54-5; C16H34, 544-76-3;CIgDa, 39756-36-0; CHD,(CHz)&HD,,
88 106-11-0.
