J. Phys. Chem. 1991, 95, 5281-5286
5281
traframework Na+ cations as illustrated in Figure 6, a deduction which is in line with the conclusionsdrawn from earlier chemical and spectroscopic measurements on similar The tungsten-oxygen bond length of 1.94 A found for the bridging W(p-O),W unit of the dimer falls in the range expected for doubly oxygen bridged tungsten(VI), e.g., 1.92 A found in W60i92-.12 (iv) The dimer structural unit illustrated in Figure 6 is fully consistent with the EXAFS derived tungsten-oxygen and tungsten-tungsten bond lengths and coordination numbers (Table 11). (v) The sequential addition of W 0 3 units to the l6(WO3).NaS6Y quantum supralattice composed of (W03)2 dimers is visualized as a continued buildup of a-cage (W0,)2 dimers, resulting in an accumulation of a-cage encapsulated dimers-ofdimers {(WO& (Figure 7) rather than cluster accretion to acage trimers (WO,), and/or tetramers (WO,),. Figwe 7. CHEM-xmodel for two orthogonally oriented, a-cage immo-
bilized ZONa-02W(p-O)2W02--NaOZ moieties, involving adjacent Na* site I1 cation six-ring anchoring sites.
A), suggests that the EXAFS data are best explained in terms of the formation of a tungsten(V1) oxide dimer (WO,), (Figure 6), occupying every a-cage of NaS6Y. (iii) The (W03)z molecular dimer unit displays terminal tungsten-dioxo bond lengths of 1.77 A, which falls in a range intermediate between those having formal bond orders of 2 (e.g. 1.69-1.70 A in W60,9s)12and li/, (e.g. 1.82 A in W04s).i'J This observation provides indirect evidence for the interaction of the terminal tungsten-dioxo groups of the (WO,), guest with ex-
Acknowledgment. We acknowledge the Natural Sciences and Engineering Research Council of Canada's Operating and Strategic Grants Programmes for generous financial support of this work. SO.expresses his gratitude to the Middle East Technical University for granting him an extended leave of absence to conduct his research at the University of Toronto. K.M. and T.B. acknowledge partial funding for this work from the donors of the Petroleum Research Fund, administered by the American Chemical Society. The operational funds for NSLS beamline X-1 1A are provided by DOE Grant DEAS0580ER10742. Sup plies of high-quality zeolites from Dr. Edith Flanigen at Union Carbide, Tarrytown, NY, are gratefully appreciated. R@m NO. W(CO)6, 14040-11-0; WO1, 1314-35-8.
Low-Frequency Single-Crystal Raman, Far- Infrared, and Inelastic Neutron Scattering Studles of Acetanilide at Low Temperature Clifford T. Johnston,* Department of Soil Science, University of Florida, Gainesville, Florida 3261 I
Stephen F. Agnew? Juergen Eckert,: LJewellyn H. Jones: Basil I. Swanson: and Clifford J. Unkefert INC-4 and P-LANSCE, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: May I I , 1990; In Final Form: January 22, 1991)
Single-crystal Raman, far-infrared (far-IR), and inelastic neutron scattering (INS) spectra of acetanilide (ACN) in the low-frequency region (20-200 cm-') were obtained as a function of temperature. At 20 K, a total of 29 low-frequency Raman-active vibrational modes were resolved and assigned to a unique symmetry species. For comparison, a total of 23 region at 20 K. Factor group analysis of ACN predicts 24 Raman-active far-IR bands were observed in the 20-200-~m-~ and 15 IR-active phonon bands. The greater-thanexpectednumber of observed bands in the low-frequency region was assigned to the presence of low-frequency internal modes that exhibit anomalous frequency-shift and line-narrowing behavior upon cooling. The most striking change occurs in the B,, (xu) polarization where a single band at 126 cm-l at 305 K splits into three distinct, well-resolved bands at 133, 142, and 152 cm-' at 20 K. INS spectra of ACN and its deuterated isatopomers demonstrate clearly that the 142-cm-I band corresponds to a methyl torsion. In addition, low-frequency bands at 104 and 189 cm-I were assigned to internal modes. In contrast to the phonon modes, the low-frequency internal modes all exhibit greater line broadening with temperature.
Introduction
The anomalous temperature dependence of the amide-1 region of crystalline acetanilide (ACN) has received considerable attention r e ~ e n t l y . ~ -In ~ ~the . ~amide-I ~ region, crystalline ACN is characterized by two strong Raman-active amide-I bands at 1665 cm-' (A,) and 1678 cm-I (Bi,), and one IR-active band at
'* P-LANSCE. INC-4.
1665 cm-I at 300 K. When ACN is cooled, a new IR- and Raman-active band appears at 1650 Cm-', which increases in (1) Sauvajol, J. L.; Almairac, R.; Morct, J.; Barthcs, M.;Ribet, J. L. 1. Rumun Spcrrosc. 1989,20, 517. 1310. (2) Blanchet, G. B.; Fincher, C. R., Jr. Phys. Rev. Lcrr. 1985, 54 (12),
(3) Scott, A. C.; Buio, I. J.; Johnston, C. T. Phys. RN. B 1989,39,12883. (4) Johnston, C. T.; Swanson, B. I. Chem. Phys. Lcrr. 1985, 114 ( 5 , 6 ) , 541.
0022-3654/91/2095-5281$02.50/0
0 1991 American Chemical Society
5282 The Journal of Physical Chemistry, Vol. 95, No. 13, 199'1 intensity at the expense of the 1665-cm-' band.'J2J3 A broad diversity of theoretical models have been proposed to account for the appearance of the novel 1650-cm-' band. Proposed explanations range from a Fermi resonance argument' to the existence of a self-trapped vibrational state similar to a Davydov soliton."J3 Although the current models are diverse, each involves a coupling mechanism between the amide-I vibrational mode and a lowfrequency optical or acoustic phonon mode. Despite the probable coupling of the amide-I mode with an external mode, relatively little data is available on the temperature dependence of the low-frequency IR- and Raman-active vibrational modes of ACN. Acetanilide is characterized by a well-defined linear network of hydrogen bonds that extend along the crystallographic b axis. ACN has been used as a model system to study polypeptide hydrogen-bonded chain structures because of the similarity between the hydrogen-bonded amide groups in ACN to more complex biological molecules (e.g., a-helical structures) and the high degree of crystal symmetry it possesses. In a picosecond CARS study of hydrogen-bonded crystals that included ACN, several low-frequency optical phonons of ACN were found to be long-lived at low temperature15with line widths on the order of 0.005 cm-I. The longest-lived optical phonon for ACN was the 39-cm-I B mode, which had a lifetime of 1164 ps at 10 K.Is These long-livd optical phonon modes have been used recently as a vibrational probe to study accumulated optical damage in ACNa6 The amide-I band itself, however, is a short-lived mode that is strongly coupled to the lattice and has typical relaxation behavior for a molecular crystal.26 In view of the interest in ACN and the importance of coupling between the amide-I vibrations and modes in the phonon region, we report here the temperature dependence and symmetry assignments of the low-energy modes. Polarized single-crystal Raman methods permit individual symmetry components to be obtained; in a molecular solid as complex as ACN this greatly reduces the number of observed vibrational bands in a given orientation. In addition, polarized single-crystal Raman spectra of ACN were obtained as a function of temperature in the 20-300 K range in order to (1) fully resolve the low-frequency bands and (2) observe any anomalous behavior related to the unusual temperature dependence of the amide-I region or to a second-order phase transition. In a previous low-frequency Raman study of ACN, Gerasimov reported a total of 27 Raman-active bands at 110 K.16 Although he tentatively observed all of the bands predicted on the basis of a factor group analysis for ACN, he did not report on the temperature dependence of the low-frequency bands. In addition, several of the low-frequency bands he observed have not been observed in more recent studies.Is Thus, we report here the temperature dependence of polarized single-crystal Raman and far-IR spectra of ACN.
(5) Wasscrman, H. J.; Ryan, R. R.; Layne, S.P. Acra Crystallogr. 1985, C41, 783.
(6) Kosic, T. J.; Hill, J. R.; Dlott, D. D. Chem. fhys. 1986, 104, 169. (7) Careri, G.; Gratton, E.; Shyamsunder, E. fhys. Rev. A 1988,37,4048. (8) Scott, A. C.; Gratton, E.;Shyamsunder, E.; Careri, G. fhys. Rev. B 1985, 32. 5551. (9) Campa, A.; Giansanti, A.; Tenenbaum, A. Phys. Rev. B 1987, 36, 4394. (10) Tenenbaum, A.; Campa, A.; Giansanti, A. fhys. Lett. A 1987,121, 126. ( 1 1 ) Eilbeck, J. C.; Lomdahl, P. S.; Scott, A. C. Phys. Rev. B 1984, 30 (8), 4703. (12) Csreri. G.; Buontempo, U.; Galluzzi, F.; Scott, A. C.; Shyamsunder, E.; Gratton, E. Phys. Rev. B 1984, 30 (8), 4689. (13) Careri, 0 . ; Buontempo, U.; Carta, F.; Gratton, E.; Scott, A. C. fhys. Rev. Lett. 1983. 51 (4), 304. (14) Taylor, A. D.; Wood,E. J.; Goldstone, J. A.; Eckert, J. Nucl. Inst. Methods fhys. Res. 1984. 221, 408-418. (IS) Kosic, T. J.; Cline, R. E., Jr.; Dlott, D. D. J . Chem. fhys. 1948, 81 ( I ] ) , 4932. (16) Gerasimov, V. P. Opt. Specrrosc. 1977, 43, 417. (17) Brown, C. J.; Corbridgc, D. E. C. Acra Crystallogr. 1954. 7, 711. (18) Brown, C. J. Acra Crystallogr. 1966, 21, 442.
Johnston et al. We also report inelastic neutron scattering (INS) spectra of ACN and its methyl-deuterated isotopomer at 10 K. INS intensities of vibrational bands are directly dependent (among other factors) on the neutron scattering cross section of the nuclei involved in the vibrational mode in question as well as on its amplitude.'' Since the cross sections for H and D differ by more than an order of magnitude, selective isotopic substitution can be of significant value in mode assignments, particularly those of large amplitude such as methyl torsions.
Experimental Section Primary standard grade (99.9% pure by assay) acetanilide (N-phenylacetamide)was obtained from G. Frederick Smith Co. Isotopic purity of products was determined by using 'H and I3C NMR spectroscopies. For growing single-crystal specimens, further purification of acetanilide was accomplished by using a multiple-passzone refiner. After several days of zone refinement, the purified material was placed in a glass tube and sealed under vacuum. The glass tube containing the zone-refined acetanilide sample was lowered through a modified Bridgeman-Stockbarger furnace at a rate of 2 cm/day. The crystal habit of acetanilide is (100) tabular; thus, cleavage was nearly perfect parallel to the (100) face, and fairly good parallel to the (001) face. The crystals used in the low-temperature, single-crystal Raman experiments were, on average, 10 mm in length along they axis, 4 mm along the z axis, and 1 mm along the x axis. Assignment of the crystallographic axes was confirmed using X-ray precession methods and a cross-polarizing microscope. Single-crystal specimens of acetanilide were mounted in an oxygen-free copper cold cell, which enclosed the crystal in an inert (He) atmosphere. The cold cell was mounted on the cold finger of an Air Products Displex refrigerator. A 180' backscattering geometry was used to collect the scattered radiation. Temperature of the cell body was determined by using a Au-chromel thermocouple. The local temperature of the acetanilide crystal was measured by determining the Stokes:anti-Stokes intensity ratios for several low-frequency bands and applying Boltzman statistics. Stokes and anti-Stokes spectra were collected in a single scan by scanning over the -200 to +200 Acm-l region continuously with neutral density filters in the -15-cm-' to +15-cm-I region to attenuate the laser line. Comparison of the temperatures determined by using the Stokes:anti-Stokes method to the thermocouple values indicated that local heating of the crystal by the laser was less than 10 K. The temperatures listed for all of the single-crystal Raman spectra were determined by using the measured Stokes:anti-Stokes ratio. Raman spectra were collected on a Spex 1403 0.75-m double monochromator interfaced to a Nicolet 1180E computer. The 514.5-nm line of a Spectra Physics Model 171 argon ion laser was used as the exciting line. The spectral slit width ranged from 2 cm-I for room-temperature scans to 0.5 cm-' for the low-temperature scans. A rotating polarizing filter and analyzer were used to collect the oriented singlecrystal Raman spectra. Far-IR spectra were collected on a Digilab FTS Model 20 FT-IR spectrometer using a 6 - ~ mmylar beamsplitter and a liquid-He-cooled Ge bolometer. Band positions, line shapes, and bandwidths for the Raman spectra were determined by using a nonlinear ieastsquares band analysis program on a DEC Vaxstation 3100 computer. Inelastic neutron scattering data were collected at 10 K on the Filter Difference Spectr~meter'~ at the Manuel Lujan, Jr. Neutron Scattering Center of Los Alamos National Laboratory by using approximately 3-4 g of polycrytsalline material of ACN and ACN (methyl-d,). This instrument has a pulsed "white" neutron beam incident on the sample. Band-pass filters are used to select from the scattered neutrons those falling within a particular energy band. The incident energy and thus the energy transfer to the sample can then be inferred from the time of flight of the neutrons. Data is then deconvoluted with maximum entropy methods,19 (19) Sivia, D. S.;Vordenvisch, P.; Silver, R. N. Nucl. Inst. Methods fhys. Res., in press.
The Journal of Physical Chemistry, Vol. 95, NO. 13, 1991 5283
Low-Frequency Vibrational Modes of Acetanilide TABLE I: Law-Frequeaey R i m a lad Fir-IR &ads of ACN
Raman
B1,
A,
21 K 35
110K 33
300K 29
,B
21 K
110K
300K
38
35
28
21 K
110K
plycryst capillary
B3,
300K
21 K
110K
300K
20K 35
39
37
34
39 42 45
52
46
40
42 48
45
far-infrared plycryst 20 K
39 49
56
53
47 58
55
58
51 59
56
50 64
64 59
68
64 65 67
68
70 74 76
74
76
68
ia
75
78
61
79 81 82 86 92
88
84
80
75
76
19 96
92
82
98 104
101
90
a2 86 92 96 98 104
107
103
95 112
110
102
101 104 107
112 113 123
1 I3
124
124 126 130
133
130 133
126
134 137 142
138
146
129
127
122
137 142 145 148 152
130
145 146 147 148 156 165 187
148
which produces a reconstruction of the scattering function with the instrumental effects removed. The energy resolution that can be achieved in this way is approximately 1-2% of the energy transfer. Results and Discussion Low-Temperature Raman, Far-IR, and INS Spectra of ACN. Acetanilide crystallizes in the orthorhombic D2115(Pbca) space group with eight molecules per unit cell; the site symmetry of each ACN molecule in the unit cell is C1.5*179'8 The crystallographic unit cell is the same as the primitive cell because the Bravais lattice is of the primitive type. Factor group analysis of ACN predicts a total of 45 external vibrations corresponding to 21 translational (1 2 Raman-active, six IR-active, and three IR-inactive) and 24 librational (12 Raman-active, nine IR-active, and three IR-inactive) symmetry species. Thus,a total of 24 Raman-active modes (six A,, six BIB,six Ba, and six B$ and 15 IR-active modes (five Blurfive B2U,and five Bs) are predicted. At 300 K, a total of 26 Raman-active bands were observed in the 20-200-cm-' region among the six polarization elements (xx, y y , zz,xy, xy, and y z ) corresponding to eight A,, seven B 1 ,six B, and five B, symmetry species (Table 1). Many of the low-frequency bands of ACN are
135
147
189
poorly resolved at 300 K due to large bandwidths that overlap with other transitions. Thus, single-crystal Raman and far-IR spectra of ACN were obtained as a function of temperature between 20 and 330 K in order to better resolve the low-frequency modes and to observe the influence of temperature on the shifts in frequency and line shape of the phonon modes. Single-crystal Raman spectra of ACN at 21 K corresponding to the z@y)z, z(xy)z,and z(xx)z polarization elements are shown in Figure 1 along with a comparison to the Raman spectrum of a polycrystalline (capillary) ACN sample. In contrast to the broad line shapes observed at 300 K, the low-temperature Raman bands are well resolved having full width at half-maximum (fwhm) values of less than 2 cm-I. The positions and symmetry assignments of the low-frequency Raman and far-IR bands obtained at different temperatures are listed in Table I. A total of 29 Raman-active bands were observed among the six polarization elements at 20 K. The modes were distributed as nine A,, eight B,,, five B2,, and seven B,, symmetry species. The low-temperature capillary Raman spectrum of ACN (Figure 1) agrees well with the single-crystal Raman spectra; however, it was not possible to observe modes below = 40 cm- due to inelastic light scattering from this sample. These results are in reasonable agreement with a previous
5284 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Johnston et al. ACN
150
100
Wave Number (cm-')
ACN (methyl-d3) Lo
20.
50.
80.
110.
140.
170.
,-t
200
Wavenumber
Figure 1. Polarized single-crystal Raman spectra of ACN corresponding
to the (xx), by),and (xy) polarization elements in the 20-200-cm-I region obtained at 20 K (Stokes:anti-Stokes temperature). For comparison, the Raman spectrum of polycrystalline ACN at 20 K is shown in the 20-200-cm-' region.
low-frequency Raman study of ACN by Gerasimov,16 who observed 27 bands at 110 K (these values are included in Table I for comparison). In comparison to the Ram;n results, a total of 23 IR-active modes were observed in the 50-200-cm-' region of the far-IR spectrum of a polycrystalline ACN sample (Table I) in comparison to the 15 predicted by group theory. The 6-pm mylar beamsplitter used for the far-IR measurements did not permit measurements below 50 cm-' to be made. Four Raman-active bands were observed below 50 cm-I at 21 K;thus, it is probable that several IR-active bands occur in this region that were not observed in this study, which would increase the total number of far-IR modes to 26 or 27 IR-active modes in the 20-200-cm-' region. Tremendous line broadening of the low-frequency bands was observed in the far-IR spectra at temperatures above 150 K. At 250 K, for example, only three very broad absorption features were found in the 5&200-cm-' region. For both the Raman and the IR spectra more bands were observed at low temperature than were predicted by group theory. These results are unusual in that generally fewer modes are observed than are predicted for crystalline molecular solids.2o There are three possible sources for the additional modes observed in the phonon region: (1) the presence of low-energy internal modes, (2) the presence of combination and/or overtone modes, or (3) the presence of a structural phase transformation at reduced temperatures resulting in splittings of existing modes or new phonon modes. Low-temperature X-ray structural studies by Wasserman et aL5 showed no evidence for a change in the DZhls space group in going from 300 to 1 13 K. In addition, studies by Careri et al.'"' of the specific heat and the lattice constants, which (20) (21)
Cramer, S. P.;Hudson, B. 1.Chem. Phys. 1W6.61 (3,1140. Careri. 0.;Compatangelo, E.;Christiansen, P. L.; Halding, J.;
Skovgaard. 0.Phys. Ser. 1986,35,64. (22) Bcllows, J. C.; Prasad. P. N. 1. Chrm. Phys. 1979, 70 (M), 1864. (23) Nielm, 0. F.;Bigio, 1. J.; Olsen, 1.; Bequier, J. M.Chrm. Phys. Lett. 1984, 132, 502. (24) Jensen, J. H.;Christiansen, P. L.; Skovgaard, 0.;Nielsen, 0. F.; Biipo, 1. J. Phys. Lett. A 1986. 117, 123. (25) Sec, for example: Howard, J.; Waddington, T. C. Adwmrs in Infrond und Ruman Sprclmceopy; Clark, R. J. H., Hater, R. E., Heyden: London, 1980 Vol. 7, Chapter 3.
=.;
150
100
Wave Number (cm-')
Figure 2. Inelastic neutron scattering (INS) spectrum of polycrystalline (top) acetanilide and (bottom) (methyl-& acetanilide at 10 K in the
maximum entropy method re~onstruction.~~
both showed monotonic changes with temperature, argue against a major structural phase change for ACN. However, Fann et a1.26recently suggested that multiple configurations for the amide group may be present at low temperature which would be insensitive to X-ray detection. The results presented here argue against a structural phase transformation, as one expects a soft mode for a second-order phase change or discontinuities in the phonon spectra in the event of a first-order phase change. The presence of overtone and combination bands cannot be ruled out. However, we expect the intensities of overtones and combinations to be low and direct comparisons of the frequencies observed at low temperature with the frequencies of possible combinations or overtones show little correspondence. The low-temperature INS spectra for ACN and its methyldeuterated isotopomer are shown in Figure 2 in the maximum entropy method reconstru~tion.'~ Assignment of bands in the M S spectrum is facilitated by noting that those modes involving large amplitude motions of nuclei with large neutron scattering cross-sections will have the highest inten~ities.~~ On this basis, the mode at 142 cm-I can be unambiguously assigned as the methyl torsion. Methyl group deuteration (bottom of Figure 2) confirms this assignment, as most of the intensity in this band disappears. It should also be noted that there is evidence for structure in the methyl torsion band including the shoulder on the low-frequency side, which could correspond to the splitting of the 142-cm-' band observed in the single-crystal Raman data to be described below. At least two additional INS bands can be seen to change significantly on methyl deuteration, those in the regions of 100 and 185 cm-'. Their assignments will be discussed in the following section. ~
~
_
_
_
_~ _ ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~
Fann, W:;Rothberg, L.; Roberson, M.;Bcnson, S.;Madey, J.; Etemad, S.; Austin, R. Phys. Rev. Lctr. 1990, 61.607. (26)
Low-Frequency Vibrational Modes of Acetanilide
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5285
-
0
20:
.
7
-
15
-
35
0
104
0 A A
133
0 :
0 8
142 152
8 A
8 5-
o b
Wavenumber
Figwe 3. Temperature dependence of the z@y)z Raman spectra of ACN in the 20-200-cm-I region of crystalline ACN between 21 and 305 K.
m Luil.il 53K
113K
149 K
209 K
305 K 140
170
Wavenunbet
Figure 4. Temperature dependence of (a) z@y)z and (b) z(xy)z Raman spectra in the 110-160.~m-~region of crystalline ACN between 20 and 300 K.
Temperature Dependence. All of the low-frequency Ramanand IR-active modes increase in frequency and narrow in line width as the temperature is reduced resulting in improved spectral resolution (Figures 3 and 4). For the Raman spectra collected at low temperature, the natural line widths of some modes are less than the band-pass of the spectrometer. The 38-cm-' band of Bl, symmetry, for example, has a picosecond CARS lifetime of 1164 ps,I5 which corresponds to a Lorentzian line width of r = 0.0045 cm-I. Kosic et aI.l5 reported measurable picosecond CARS lifetimes (Le., >10 ps) for bands at 38, 39, 52, 58.82, and 104 cm-'. The temperature dependence of the line widths of several low-frequency modes was analyzed by using the approach of Bellows and Prasad.22 They noted that the line widths for phonon modes of naphthalene were determined primarily, not by T2 processes, but by TIprocesses involving energy exchange. They were able to fit the temperature dependence of the widths by a
8
0 0
A
* , , , e . , , e , ,e
,
,
, ,
0 ,
,
O , , ,
,
@ ,
,
O
,
5286 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
142 '1,
0 m I
I25 0
'
'
.
'
'
50
.
'
'
' 100
'
'
'
'
'
'
'
'
' '
'
' ' ' '
'
0
'
150 200 250 Temperature (K)
'
'
'
'
' '
300
.'
350
Figure 6. Shift in frequencyof the 133- (B,J, 142- (BIJ, 151- (AB),and 152-cm-' (BIJ bands as a function of temperature between 21 and 305
K. Positions of the bands were determined by using a nonlinear leastsquares band analysis program, which fit the individual components to a line shape made up of Gaussian and Lorentzian distributions. TABLE II: Temperature-InducedFrequency-Shift Values for Selected B.ads of Crvscrlline Acetanilide
band, cm-l 35 68 92 104 142
152 189
io-2(avianp cm-' K-I -2.4 -4.1 -7.5 -5.1 -5.8 -8.4 -3.6
io*(avianpiv21K, cm-l K-' -6.7 -6.1 -8.2 -4.9 -4.1 -5.6 -1.9
new band at 152 cm-' in the z(xy)z spectrum was the 151-cm-' band from the z@y)r spectrum leaking into the z(xy)z spectrum, then a proportional increase in intensity of all the zcVy)z modes must occur. The dominant band in the z@y)z spectrum is the 104-cm-' band. This band has negligible intensity in the x(xy)z spectrum at 21 K;thus, the 152-cm-' band in the z(xy)z spectrum of B,, symmetry is distinct from the 151-cm-' band in the z@y)z spectrum of A, symmetry. A similar approach was used to verify that the B,, spectra were not contaminated by other symmetry elements (e.g., ( x x ) , b y ) , (zz), ( x z ) and (yz)). Thus, the new bands at 133 and 152 cm-' have B,,symmetry and the assignment for these bands, at present, is unknown. In a recent picosecond-infrared excitation study of ACN, Fann et alez6suggested that muliple configurations for the amide groups that are separated by a thermal barrier are present. This hypothesis may be related to the anomalous splitting of the 133-, 142-, and 151-cm-I bands. When the sample is cooled from 298 K to 87 K,the volume of the unit cell of ACN decreases from 1486"J8 to 1420 A3.s Thus, the increase in frequency observed for all of the low-frequency bands, characteristic of hydrogen-bonded organic crystals, results primarily from the thermal contraction of the crystal lattice at low temperature.= The shift with temperature was fairly linear in the 50-250 K region. The temperature-induced frequency shift values ( 8 ~ / 8 Tin) ~this temperature range varied from -0.023 cm-' K-' for the 354x11-' band to -0.084 cm-'K-' for the 151-cm-l (A8) band (Table 11). Fractional frequency shift values were obtained )~ by the low-temperature frequency by dividing the ( 8 ~ / 8 7 'values of the band. These fractional values ranged from -4.1 X lo4 to -8.2 X IO4 K-I for the external modes, whereas the value for the 189-cm-' band was -1.9 X lo4 K-'(Table 11). It is interesting to note that the fractional values for the 104- and 142-cm-I bands were the lowest of all the low-frequency bands with the exception of the 189-cm-' band since some internal character has been ascribed to both of these bands. The low fractional increase in frequency of the 189p21K-cm-1 (A4) band (Table 11) is in good agreement with the previous assignment of this band to an internal mcde.4J6 Smaller temperture-induced frequency shifts would be anticipated for internal modes in comparison to those of the phonon modes because the molecular geometry is assumed to be less affected by temperature than is the crystal structure. In addition, this band is close in
Johnston et al. frequency to a broad band a t 180 cm-' observed in liquid ACN (mp 388 K) heated to 393 K. A similar band occurs at 190 cm-l in the Raman spectrum of liquid N-methylacetamide (300 K)that has been assigned to a torsional motion around the C-N bond.23 Sauvajol et al.' recently reported that the 104-cm-' band was strongly perturbed by deuteration. Upon complete deuteration of ACN, the 104-cm-' band is split into two components that are characterized by much larger frequency shifts than is the case for the normal isotopic species. In a previous Raman study of ACN? for example, it was suggested that the 104-cm-l band may correspond to an internal mode on the basis of its anomalous frequency-shift and linenarrowing behavior. Internal modes near 100 cm-l have been observed for a number of diverse hydrogenbonded organic species including formamide, N-methylformamide, guanosine-5'-monophosphate and tRNA. In the case of Nmethylacetamide, Nielson et a1.23,24assigned this band to an out-of-chain motion involving atoms in the hydrogen bond (Le., the amide group). The INS spectra in the regions corresponding to the Raman bands of 189 and 104 cm-' are quite clearly sensitive to methyl deuteration. For the latter region it should be also be noted that the spectrum of the deuteriomethyl compound is likely to contain the CD3 torsion. A simple analysis using a three-fold rotational potential for the methyl group with the observed transition at 142 cm-' suggests a barrier height of approximately 1.5 kcal/mol. When the energy levels and potential are scaled by the respective rotational constants, the torsion for CD3 would in fact occur at 104 cm-I. The intensity changes upon methyl deuteration in this region would therefore be even more pronounced were it not for the shift of the methyl torsional band into this region. As far as the assignment of the 104- and 189-cm-l bands for the normal isotopic species is concerned, we can conclude from the INS data that both may be low-frequency internal modes that either include motion of the methyl group or are coupled to modes of the latter. This intra- or intermolecular vibrational coupling can be elucidated by the INS studies of the ACN isotopomers that are currently in progress.
Conclusions The substantial line-narrowing behavior of the low-energy Raman and far-IR modes in the 20-200-cm-I region a t low temperature (ca. 20 K) resulted in improved spectral resolution, which permitted all of the low-frequency bands to be resolved and assigned to a unique symmetry species. The positions and symmetry assignments presented here are in good agreement with those of an earlier study of ACN conducted at an intermediate temperature. A total of 29 Raman-active and 23 IR-active bands occur at 20 K, which compares to values of 24 and 15, respectively, predicted by factor group analysis of ACN. We attribute the greater-than-expected number of low-energy bands to the presence of low-energy internal bands. Specifically, the 104-, 142-, and 189-cm-l bands exhibited line-narrowing and frequency-shift behavior that was distinct from that of other low-frequency bands. The assignment of the 189%21K-~m-I band to an internal mode is confirmed on the basis of its temperature behavior, INS data, and isotopic shift values, which agrees with the previous assignment for this band.I6 Our INS data on the ACN isotopomers clearly support the assignment of the 142T-21K-~m-I band to a methyl torsion (internal) mode. Finally, the anomalous splitting of the 126T130sK-~m-1 band into three distinct, well-resolved components at 133, 142, and 152 cm-I, when the sample was cooled to 20 K, was observed in the z(xy)z spectrum, and also reflected in the INS data. The origin of this splitting and the assignment of these bands is currently unknown. Acknowledgment. We thank P. Vorderwisch for assistance with collection of the INS data on ACN. This work has benefited from the use of facilities at the Manual Lujan, Jr. Neutron Scattering Center, a national user facility funded as such by the DOE/Office of Basic Energy Sciences. Registry No. ACN, 103-84-4; neutron, 12586-31-1; deuterium, 1782-39-0.
