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Raman studies of solid hydrogen cyanide (HCN, DCN) and of HCN argon matrices
Spectrochimica Act&Vol.49A.No.2. pp.191-197. 1993
05~#39/93
$6.00+ 0.00
@ 1993Pcrgamon
Printed in Great Britain
Press L&l
Raman studies of solid hydrogen cyanide (HCN, DCN) and of HCN argon matrices B. MUELLERand H. D. LUTZ UniversitHt Siegen, Anorganische Chemie I
and J.
HERMELING
and E. KN&INGER*
Universitlt Siegen, Physikahsche Chemie IV, Postfach 101240, W-5900 Siegen, Germany (Receioed 27 April 1992; accepted 11 June 1992) Abstrwt-Raman spectra of single crystal-like oriented solid HCN and DCN, as well as of HCN molecules and clusters in argon matrices, have been recorded and analysed with respect to both so far uninterpreted spectral features reported in the literature and to the applicability of Raman experiments for studying molecular clusters in inert gas matrices. The main results obtained are: (i) splitting of both the intramolecular bending modes and librations of solid HCN has to be interpreted in terms of TO/LO splitting rather than site group splitting; (ii) the nature of the phase transition at 170 K reported as f!6 to oI6 must be doubted; and (iii) the previous results of HCN(DCN)/Ar matrices obtained by IR studies are confirmed.
INTRODUCTION MOLECULAR clusters represent transition states between free molecules and condensed phases. Therefore, clustering of molecules, e.g. in molecular beams or in matrices of solid inert gases, has extensively been studied in the last two decades. For rare gas matrices, the main technique employed is IR spectroscopy [l, 21. Inelastic neutron scattering (INS) [3] and Raman spectroscopic measurements are scarce [l, 21. Within the group of condensed phases with hydrogen bonds as prevailing intermolecular interaction, hydrogen cyanide was used frequently for case studies [4-g]. Thereby, linear clusters (HCN), with 2< n <6 have been observed. Crystalline HCN was studied by IR, Raman, and inelastic neutron scattering experiments [lo-131. However, some spectral features, e.g. bands additional to those predicted by group theory, are not yet understood. In order to answer the related questions, we performed Raman studies on both solid hydrogen cyanide and HCN(DCN) trapped in a solid argon matrix.
EXPERIMENTAL
Hydrogen cyanide (HCN and DCN) was prepared by reaction of NaCN with H2S04 (D2SOa)[7]. The purity was checked by IR analyses. In the case of neat HCN and DCN, the gaseous compound was condensed into a glass tube, which was subsequently sealed. For oriented crystallization, the sealed glass tubes were slowly cooled down to 245 K and annealed at this temperature for 10 h. The c-axis of the single crystal-like solidified sample was oriented parallel to the axis of the tube. For preparing solid argon matrices, argon (purity 4.8) and HCN(DCN) were mixed in an evacuated (10m3Pa) glass flask (2.5 1). The partial pressure of the gases and the amount of the condensed material were established by standard manometric procedures (Barocel 600 A Datametrics capacity monometer). The Raman spectra were recorded on a Dilor Omars 89 multichannel detector Raman spectrograph with the usual right angle geometry (spectral slit width 3 cm-‘). The 514.5 nm line of an Ar+ ion laser was used for excitation (power
B. MUELLERetal.
192
Fig. 1. Cryostat used for the Raman experiment (a, aluminium mirror; b, quartz glass window; c, outer shroud; d, connection of the pump; e, siphon-shaft for helium).
modified (see Fig. 1) in such a manner that the HCN/Ar mixture could be condensed through one of the five windows. As the cryogenic sample support an aluminium mirror, vacuum-metallixed with gold, was employed. In the case of neat hydrogen cyanide, a block of copper was used as a temperature buffer. The amounts of condensed HCNlAr mixtures are given in Table 1. The condensation rate was 0.8 mmol h-l; the temperature of the mirror 8 K. For more details see Ref. [15].
RESULTS Raman spectra of condensed and crystalline hydrogen cyanide are shown in Figs 2-3. The frequencies observed are given in Table 2. The spectra of crystalline hydrogen cyanide obtained resemble those reported in Refs [ll, 121. However, additional diiec-
Table 1. Composition of the HCNlAr matrices studied (for more details see Ref. [lS]) HCNlAr 1500 1:200 1:lOo Neat HCN
Amount of deposited HCN(g) 2.2 x 5.5 x 1.1 x 3.6 x
10-4 10-4 10-3 lo-’
Raman studies of HCN, DCN and HCN argon matrices
193
dyx)b dxx)b
a(xx)b
J rl
n60
2060
A
1
900
600
y
250
150
.
*
3160
3060
”
250
150cni1
Fig.2. Raman sPectra of (single crystal-like)crystallineHCN (glass tube) at 65 and 185K, respectively.Polarixabilitytensor elementsxx, yy, zz refer to species Al (C,,, Czo); that of xy to B2 (C,,), A2 (C,,); xz to E, B,; audyz to E, B2 respectively. a, b, and c refer to the crystal axes of solid HCN. Some more spectra are shown in Ref. [I$
tional properties of the Raman bands (due to the single crystal-like samples) are established. These permit the nature of the bands which are not predicted by group theory to be explained. Furthermore, the nature of the phase transition claimed in the literature [13,16] appears to be doubtful (see below). The spectra of condensed HCN reveal a considerable degree of disorder, which disappears on annealing (see Fig. 4). The spectra of HCN(DCN)/Ar matrices are shown in Fig. 5. The assignment of the bands observed as well as the interpretation with respect to monomers and oligomers of HCN are given in Table 3. The results obtained resemble those of previous IR studies [591 and, in addition, reveal IR inactive, previously not observed, intermolecular modes of HCN clusters.
65K 1886
162
I
-J**
b(xrG
b(z2)6
L
b(xx)6
185K
65K
qyx)b
66
.dYx)b
.
I(zx)b --
-4 aiyzb
b(xx) 6
b(xz)6
Fig. 3. Ramanspectraof crystallineDCN at 65 and 185K, n%Pcctively(for further explanations see Fig.2).
184
B. MILLERer al. Table2. Raman bands (cm-‘) of crystalline HCN and DCN above and below the phase transition reported HCN 65 K %H.
D)
DCN 185K
vcN
3125 2099,2106(?)
V&
2066
2064
826,846 171,247
816,835 156,231
+J. R,.
wN* WN*
3140 2o!?J
65K
185K
2547 1886
2553 1890
647,654 162,233
648,646 147,210
* TO and LO phonon modes, see also Ref. [12].
DISCUSSION
In the case of free, gaseous HCN molecules, there are three normal modes, i.e. two stretching vibrations, vcH and vcN, and one degenerate HCN bending mode. The frequencies observed, viz. 3311(2630), 2097 (1925), and 712 (569) cm-’ (data of DCN in parentheses) [18], are shifted to higher and lower values, respectively, in condensed phases owing to intermolecular interaction, in particular hydrogen bonding. Solid hydrogen cyanide was reported to be dimorphic. Accordingly, it crystallizes below 170 K in the orthorhombic space group Zmm2-C$ with 2 = 2, HCN $6, above
t
636
006
b 65 K
C
160K 64a d 65K
6’S K
Fig. 4. Raman spectra of hydrogen cyanide condensed on an aluminium mirror (a, condensed at 15 K; b, a annealed at 65 K; c, a annealed at 150 K; d, condensed at 65 K; e, d annealed at 150 K. Annealing time 3Omin in each case; measuring temperature as given in the figure; asterisks, plasma lines).
Raman studies of HCN, DCN and HCN argon matrices
195
2087
a
f
2200
2100
”
250
150
cm-
Fig. 5. Raman spectra of HCNlAr matrices (a, 1:5OD,unannealed; b, l:uw), tmanneaied; c, b annealed at 30 K for 30 min; d, b annealed at 30 K for 60 min; e, 1:lOOunannealed; f, e annealed at 30 K for 30 min. Measuring temperature 10 K each).
this temperature in the tetragonal space group Mmm-Cf, (2 = 2), HCN fI6 [16]. Group theory treatment yields [ll]: HCN 016: ... r = 2A1 (v~, v& +
2B@, R)+ 2&(6,R)
HCN t16: .. l-= 2AI(vcH, va) + 2E(6,
R).
Table 3. CN stretching modes of HCN and (HCN), clusters trapped in Ar matrix (cm-‘) Bonding
Raman
IR [9]
1
(4
2097
2093
2897
2097
2
ii;
2091 2115
2094 2113
2089 2115
2085 2108
3
;:;
2090 2107 2115
2082 2095 2110
>3
1
2889 2104 2115
2081 20894098 2110
Species (n)
(c)
C
U(A) 49:2-D
Calculated [9,17]
(a) Like HCN, ....... ...... . HCN+..HCN, .:..... etc. (b) Like HCN...HCN, .. ... .. . *. HCN, etc. HCN ... .. . **. HCN - - -HCN, etc. (c) Ltke HCN - . . HCN
B. MILLER et al.
1%
RTO ’
teXh 170-
0 x
loo-
-240
0 *
0
* 1!50-
230
: 14Oi 50
100
150
200
tK1
0
1-920 250
Fig. 6. Temperature dependence of the HCN librational modes RTOand RLO(for temperature dependence of the halfwidths see Ref. [PSI).
All these modes are both IR and Raman allowed because of the acentric space groups. In the Raman spectra both the respective transversal (TO) and longitudinal optic (LO) phonon modes should be observed. The Raman spectra obtained clearly show two scattering peaks in the bending as well as in the librational mode region, that is, for both HCN polymorphs. On the other hand, only one signal for the two stretching modes of species Al (see Figs 2 and 3) is observed. Therefore, the question arises whether the splitting of the modes of species E, and B1 and B2, respectively, is due to site group splitting. In the case of HCN t16 this would necessarily imply lower local symmetry than that according to Z4mm. Alternatively, both the TO and LO vibrations should be observed as claimed by PEZOLET and SAVOIE[12]. The single-crystal spectra, however, clearly reveal that site-group splitting is negligible. This is evidenced by the spectra recorded with b(xz)b and a(yz)b measuring geometries (see Fig. 3). On the other hand, the spectra with a(zy)d or b(xz)b measuring geometries, for which only TO modes are allowed [19], exhibit only one peak, namely the one at lower frequency. Therefore, the reasoning based on the observation of oblique phonons in the IR spectra given in Ref. [ll] is confirmed. The TO/LO splittings observed rank as R > 6 > v. This behaviour is likewise found for other crystals with polar linear molecular units as OH- ions (see, for example, Ref. [20]). The absence of site group splitting in the case of both r&., and RHCN in the lowtemperature polymorph HCN 016, . .. however, cannot be realized. It would be expected that the libration R, as well as the bending mode of species B2 are shifted to lower wavenumbers compared to Ry and d(B,) by at least 20 cm-’ with regard to the relation of the lattice constants a and b, i.e. 413 and 485 pm, respectively. Actually, there is no understandable reason for the phase transition HCN tI6+ HCN 016 claimed Ref. [Ml. At least the IR and Raman spectra do not give any evidence for the’kxistence of this phase transition. The spectra below and above 170 K are very similar as already mentioned in Refs [ll, 121 (see Fig. 6 and Ref. [El). The CH (and CD) stretching modes are shifted to lower wavenumbers compared to those of free HCN(DCN) as is expected from the relatively strong hydrogen bonds of solid hydrogen cyanide. The low-energy shift of these bands with decrease in temperature is consistent therewith. The changes of v cN compared to free HCN(DCN) are only small. The high-energy shift of the bending mode is caused by the presence of H-bonds as well as the lattice potential. For isotopic effects, e.g. different strength of H- and D-bonds (see Ref. [21]). Acknowledgementr-The financial supportof Deutsche Forschungs semeinschaft and of Fords der Chemischen Industrie is gratefully acknowledged.
Raman studies of HCN, DCN and HCN argon matrices
197
REFERENCES
[l] H. E. Hallam (Ed.), VibrationalSpecfroscopy of Trapped Species. Wiley, London (1973). [2] L. Andrews and M. Moskovits (Eds), ChemirQ and Physics of Matrix-ZsoZated Specks. North Holland, Amsterdam (1989). [3] W. Langel, Spectrochim. Acta 48A. 405 (1992). [4] A. Givan and A. Loewenschuss, 1. Mofec. Strucr. 98,231 (1983). [5] C. M. King and E. R. Nixon, J. Chem. Phys. 48, 1685 (1968). [6] J. Pacansky, J. Phys. Gem. 81, 2240 (1977). [7] E. Kniizinger and R. Wittenbeck, Ber. Bunsenges. Phys. Chem. 86,742 (1982). [8] E. Kn&inger, H. Kollhoff and W. Langel, 1. Chem. Phys. 85,48&U (1986). [9] W. Lange], H. Kohhoff and E. Kn&inger, J. Chcm. Phys. 90.3430 (1989). *[lo] R. E. Hoffman and D. F. Hornig, J. Chem. Phys. 17, 1163 (1949). [ll] M. Pezolet and R. Savoie, Con. I. Chem. 47,304l (1969). [12] M. Pezolet and R. Savoie, Con. Spectrosc. 17,39 (1972). [13] K. Aoki, B. J. Baer, H. C. Cynn and M. Nicol, Molec. Syst. High Pressure, Proc. Archimedes Workshop Mol. Solids Pressure, 2nd, 1990,283 (1991): CA, 115,81086a. [14] J. Julien and C. Hirlimann. J. Raman Spectrosc. 9,62 (1980). [15] B. Mtiller, Thesis, University of Siegen (1990). [16] W. J. Dulmage and W. N. Lipcomb, Acta Crysfafiogr. 4,330 (1951). [17] M. Kofranek, A. Karpfen and H. Lischka, Chem. Phys. 113,53 (1987). [18] H. C. Allen, E. D. Tidewell and E. K. Plyler, J. C/rem. Phys. 25,302 (1956). [19] H. D. Lutz, E. Alici and Th. Kellersohn, 1. Raman Spectrosc. 21,387 (1990). [20] P. Lagarde, M. A. H. Nerenberg and Y. Farge, Phys. Rev. 8B, 1731 (1973). [21] H. B. Friedrich and P. F. Krause, J. Chem. Phys. 59,4942 (1973).
05~#39/93
$6.00+ 0.00
@ 1993Pcrgamon
Printed in Great Britain
Press L&l
Raman studies of solid hydrogen cyanide (HCN, DCN) and of HCN argon matrices B. MUELLERand H. D. LUTZ UniversitHt Siegen, Anorganische Chemie I
and J.
HERMELING
and E. KN&INGER*
Universitlt Siegen, Physikahsche Chemie IV, Postfach 101240, W-5900 Siegen, Germany (Receioed 27 April 1992; accepted 11 June 1992) Abstrwt-Raman spectra of single crystal-like oriented solid HCN and DCN, as well as of HCN molecules and clusters in argon matrices, have been recorded and analysed with respect to both so far uninterpreted spectral features reported in the literature and to the applicability of Raman experiments for studying molecular clusters in inert gas matrices. The main results obtained are: (i) splitting of both the intramolecular bending modes and librations of solid HCN has to be interpreted in terms of TO/LO splitting rather than site group splitting; (ii) the nature of the phase transition at 170 K reported as f!6 to oI6 must be doubted; and (iii) the previous results of HCN(DCN)/Ar matrices obtained by IR studies are confirmed.
INTRODUCTION MOLECULAR clusters represent transition states between free molecules and condensed phases. Therefore, clustering of molecules, e.g. in molecular beams or in matrices of solid inert gases, has extensively been studied in the last two decades. For rare gas matrices, the main technique employed is IR spectroscopy [l, 21. Inelastic neutron scattering (INS) [3] and Raman spectroscopic measurements are scarce [l, 21. Within the group of condensed phases with hydrogen bonds as prevailing intermolecular interaction, hydrogen cyanide was used frequently for case studies [4-g]. Thereby, linear clusters (HCN), with 2< n <6 have been observed. Crystalline HCN was studied by IR, Raman, and inelastic neutron scattering experiments [lo-131. However, some spectral features, e.g. bands additional to those predicted by group theory, are not yet understood. In order to answer the related questions, we performed Raman studies on both solid hydrogen cyanide and HCN(DCN) trapped in a solid argon matrix.
EXPERIMENTAL
Hydrogen cyanide (HCN and DCN) was prepared by reaction of NaCN with H2S04 (D2SOa)[7]. The purity was checked by IR analyses. In the case of neat HCN and DCN, the gaseous compound was condensed into a glass tube, which was subsequently sealed. For oriented crystallization, the sealed glass tubes were slowly cooled down to 245 K and annealed at this temperature for 10 h. The c-axis of the single crystal-like solidified sample was oriented parallel to the axis of the tube. For preparing solid argon matrices, argon (purity 4.8) and HCN(DCN) were mixed in an evacuated (10m3Pa) glass flask (2.5 1). The partial pressure of the gases and the amount of the condensed material were established by standard manometric procedures (Barocel 600 A Datametrics capacity monometer). The Raman spectra were recorded on a Dilor Omars 89 multichannel detector Raman spectrograph with the usual right angle geometry (spectral slit width 3 cm-‘). The 514.5 nm line of an Ar+ ion laser was used for excitation (power
B. MUELLERetal.
192
Fig. 1. Cryostat used for the Raman experiment (a, aluminium mirror; b, quartz glass window; c, outer shroud; d, connection of the pump; e, siphon-shaft for helium).
modified (see Fig. 1) in such a manner that the HCN/Ar mixture could be condensed through one of the five windows. As the cryogenic sample support an aluminium mirror, vacuum-metallixed with gold, was employed. In the case of neat hydrogen cyanide, a block of copper was used as a temperature buffer. The amounts of condensed HCNlAr mixtures are given in Table 1. The condensation rate was 0.8 mmol h-l; the temperature of the mirror 8 K. For more details see Ref. [15].
RESULTS Raman spectra of condensed and crystalline hydrogen cyanide are shown in Figs 2-3. The frequencies observed are given in Table 2. The spectra of crystalline hydrogen cyanide obtained resemble those reported in Refs [ll, 121. However, additional diiec-
Table 1. Composition of the HCNlAr matrices studied (for more details see Ref. [lS]) HCNlAr 1500 1:200 1:lOo Neat HCN
Amount of deposited HCN(g) 2.2 x 5.5 x 1.1 x 3.6 x
10-4 10-4 10-3 lo-’
Raman studies of HCN, DCN and HCN argon matrices
193
dyx)b dxx)b
a(xx)b
J rl
n60
2060
A
1
900
600
y
250
150
.
*
3160
3060
”
250
150cni1
Fig.2. Raman sPectra of (single crystal-like)crystallineHCN (glass tube) at 65 and 185K, respectively.Polarixabilitytensor elementsxx, yy, zz refer to species Al (C,,, Czo); that of xy to B2 (C,,), A2 (C,,); xz to E, B,; audyz to E, B2 respectively. a, b, and c refer to the crystal axes of solid HCN. Some more spectra are shown in Ref. [I$
tional properties of the Raman bands (due to the single crystal-like samples) are established. These permit the nature of the bands which are not predicted by group theory to be explained. Furthermore, the nature of the phase transition claimed in the literature [13,16] appears to be doubtful (see below). The spectra of condensed HCN reveal a considerable degree of disorder, which disappears on annealing (see Fig. 4). The spectra of HCN(DCN)/Ar matrices are shown in Fig. 5. The assignment of the bands observed as well as the interpretation with respect to monomers and oligomers of HCN are given in Table 3. The results obtained resemble those of previous IR studies [591 and, in addition, reveal IR inactive, previously not observed, intermolecular modes of HCN clusters.
65K 1886
162
I
-J**
b(xrG
b(z2)6
L
b(xx)6
185K
65K
qyx)b
66
.dYx)b
.
I(zx)b --
-4 aiyzb
b(xx) 6
b(xz)6
Fig. 3. Ramanspectraof crystallineDCN at 65 and 185K, n%Pcctively(for further explanations see Fig.2).
184
B. MILLERer al. Table2. Raman bands (cm-‘) of crystalline HCN and DCN above and below the phase transition reported HCN 65 K %H.
D)
DCN 185K
vcN
3125 2099,2106(?)
V&
2066
2064
826,846 171,247
816,835 156,231
+J. R,.
wN* WN*
3140 2o!?J
65K
185K
2547 1886
2553 1890
647,654 162,233
648,646 147,210
* TO and LO phonon modes, see also Ref. [12].
DISCUSSION
In the case of free, gaseous HCN molecules, there are three normal modes, i.e. two stretching vibrations, vcH and vcN, and one degenerate HCN bending mode. The frequencies observed, viz. 3311(2630), 2097 (1925), and 712 (569) cm-’ (data of DCN in parentheses) [18], are shifted to higher and lower values, respectively, in condensed phases owing to intermolecular interaction, in particular hydrogen bonding. Solid hydrogen cyanide was reported to be dimorphic. Accordingly, it crystallizes below 170 K in the orthorhombic space group Zmm2-C$ with 2 = 2, HCN $6, above
t
636
006
b 65 K
C
160K 64a d 65K
6’S K
Fig. 4. Raman spectra of hydrogen cyanide condensed on an aluminium mirror (a, condensed at 15 K; b, a annealed at 65 K; c, a annealed at 150 K; d, condensed at 65 K; e, d annealed at 150 K. Annealing time 3Omin in each case; measuring temperature as given in the figure; asterisks, plasma lines).
Raman studies of HCN, DCN and HCN argon matrices
195
2087
a
f
2200
2100
”
250
150
cm-
Fig. 5. Raman spectra of HCNlAr matrices (a, 1:5OD,unannealed; b, l:uw), tmanneaied; c, b annealed at 30 K for 30 min; d, b annealed at 30 K for 60 min; e, 1:lOOunannealed; f, e annealed at 30 K for 30 min. Measuring temperature 10 K each).
this temperature in the tetragonal space group Mmm-Cf, (2 = 2), HCN fI6 [16]. Group theory treatment yields [ll]: HCN 016: ... r = 2A1 (v~, v& +
2B@, R)+ 2&(6,R)
HCN t16: .. l-= 2AI(vcH, va) + 2E(6,
R).
Table 3. CN stretching modes of HCN and (HCN), clusters trapped in Ar matrix (cm-‘) Bonding
Raman
IR [9]
1
(4
2097
2093
2897
2097
2
ii;
2091 2115
2094 2113
2089 2115
2085 2108
3
;:;
2090 2107 2115
2082 2095 2110
>3
1
2889 2104 2115
2081 20894098 2110
Species (n)
(c)
C
U(A) 49:2-D
Calculated [9,17]
(a) Like HCN, ....... ...... . HCN+..HCN, .:..... etc. (b) Like HCN...HCN, .. ... .. . *. HCN, etc. HCN ... .. . **. HCN - - -HCN, etc. (c) Ltke HCN - . . HCN
B. MILLER et al.
1%
RTO ’
teXh 170-
0 x
loo-
-240
0 *
0
* 1!50-
230
: 14Oi 50
100
150
200
tK1
0
1-920 250
Fig. 6. Temperature dependence of the HCN librational modes RTOand RLO(for temperature dependence of the halfwidths see Ref. [PSI).
All these modes are both IR and Raman allowed because of the acentric space groups. In the Raman spectra both the respective transversal (TO) and longitudinal optic (LO) phonon modes should be observed. The Raman spectra obtained clearly show two scattering peaks in the bending as well as in the librational mode region, that is, for both HCN polymorphs. On the other hand, only one signal for the two stretching modes of species Al (see Figs 2 and 3) is observed. Therefore, the question arises whether the splitting of the modes of species E, and B1 and B2, respectively, is due to site group splitting. In the case of HCN t16 this would necessarily imply lower local symmetry than that according to Z4mm. Alternatively, both the TO and LO vibrations should be observed as claimed by PEZOLET and SAVOIE[12]. The single-crystal spectra, however, clearly reveal that site-group splitting is negligible. This is evidenced by the spectra recorded with b(xz)b and a(yz)b measuring geometries (see Fig. 3). On the other hand, the spectra with a(zy)d or b(xz)b measuring geometries, for which only TO modes are allowed [19], exhibit only one peak, namely the one at lower frequency. Therefore, the reasoning based on the observation of oblique phonons in the IR spectra given in Ref. [ll] is confirmed. The TO/LO splittings observed rank as R > 6 > v. This behaviour is likewise found for other crystals with polar linear molecular units as OH- ions (see, for example, Ref. [20]). The absence of site group splitting in the case of both r&., and RHCN in the lowtemperature polymorph HCN 016, . .. however, cannot be realized. It would be expected that the libration R, as well as the bending mode of species B2 are shifted to lower wavenumbers compared to Ry and d(B,) by at least 20 cm-’ with regard to the relation of the lattice constants a and b, i.e. 413 and 485 pm, respectively. Actually, there is no understandable reason for the phase transition HCN tI6+ HCN 016 claimed Ref. [Ml. At least the IR and Raman spectra do not give any evidence for the’kxistence of this phase transition. The spectra below and above 170 K are very similar as already mentioned in Refs [ll, 121 (see Fig. 6 and Ref. [El). The CH (and CD) stretching modes are shifted to lower wavenumbers compared to those of free HCN(DCN) as is expected from the relatively strong hydrogen bonds of solid hydrogen cyanide. The low-energy shift of these bands with decrease in temperature is consistent therewith. The changes of v cN compared to free HCN(DCN) are only small. The high-energy shift of the bending mode is caused by the presence of H-bonds as well as the lattice potential. For isotopic effects, e.g. different strength of H- and D-bonds (see Ref. [21]). Acknowledgementr-The financial supportof Deutsche Forschungs semeinschaft and of Fords der Chemischen Industrie is gratefully acknowledged.
Raman studies of HCN, DCN and HCN argon matrices
197
REFERENCES
[l] H. E. Hallam (Ed.), VibrationalSpecfroscopy of Trapped Species. Wiley, London (1973). [2] L. Andrews and M. Moskovits (Eds), ChemirQ and Physics of Matrix-ZsoZated Specks. North Holland, Amsterdam (1989). [3] W. Langel, Spectrochim. Acta 48A. 405 (1992). [4] A. Givan and A. Loewenschuss, 1. Mofec. Strucr. 98,231 (1983). [5] C. M. King and E. R. Nixon, J. Chem. Phys. 48, 1685 (1968). [6] J. Pacansky, J. Phys. Gem. 81, 2240 (1977). [7] E. Kniizinger and R. Wittenbeck, Ber. Bunsenges. Phys. Chem. 86,742 (1982). [8] E. Kn&inger, H. Kollhoff and W. Langel, 1. Chem. Phys. 85,48&U (1986). [9] W. Lange], H. Kohhoff and E. Kn&inger, J. Chcm. Phys. 90.3430 (1989). *[lo] R. E. Hoffman and D. F. Hornig, J. Chem. Phys. 17, 1163 (1949). [ll] M. Pezolet and R. Savoie, Con. I. Chem. 47,304l (1969). [12] M. Pezolet and R. Savoie, Con. Spectrosc. 17,39 (1972). [13] K. Aoki, B. J. Baer, H. C. Cynn and M. Nicol, Molec. Syst. High Pressure, Proc. Archimedes Workshop Mol. Solids Pressure, 2nd, 1990,283 (1991): CA, 115,81086a. [14] J. Julien and C. Hirlimann. J. Raman Spectrosc. 9,62 (1980). [15] B. Mtiller, Thesis, University of Siegen (1990). [16] W. J. Dulmage and W. N. Lipcomb, Acta Crysfafiogr. 4,330 (1951). [17] M. Kofranek, A. Karpfen and H. Lischka, Chem. Phys. 113,53 (1987). [18] H. C. Allen, E. D. Tidewell and E. K. Plyler, J. C/rem. Phys. 25,302 (1956). [19] H. D. Lutz, E. Alici and Th. Kellersohn, 1. Raman Spectrosc. 21,387 (1990). [20] P. Lagarde, M. A. H. Nerenberg and Y. Farge, Phys. Rev. 8B, 1731 (1973). [21] H. B. Friedrich and P. F. Krause, J. Chem. Phys. 59,4942 (1973).