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Vapour-liquid frequency shifts in some substituted methanes
Spectrochimica$cta, 1963,Vol. 19, pp. 1979to 1987. Pergamon Press Ltd. Printed in Northern Ireland
Vapour-liquid frequency shifts in some substituted methanes R. E. KA~ARISE U.S. Naval Research Laboratory,
Washington
25, D.C.
(Received 12 June 1963) Abstract-The frequency shifts which occur in going from the vapor phase to the pure liquid have been observed for methyl chloride, methyl bromide, methyl iodide, methyl cyanide, methylene chloride and methylene bromide. In addition, vapor to carbon tetrachloride solution frequency shifts were observed for the three methyl halides. The observed shifts were independent of the symmetry of the vibrational mode and assume a wide range of values within an A comparison of shifts between related molecules shows well-defined individual molecule. patterns. For example, the ys(e) fundamental in the methyl halide exhibits a small positive shift in all three molecules, whereas ~~(a,), the C-X stretching mode, is characterized by rather large negative shifts. Finally, a good correlation between a,u/&S’,the dipole moment derivative with respect to symmetry coordinate, and the vapor-liquid frequency shifts is observed for the methyl halide fundamentals. INTRODUCTION THE qualitative nature of the changes which occur in infrared absorption bands in going from the vapor to the liquid state is reasonably well-established [l]. In particular, the rotational fine structure, if present in the vapour state spectrum, is usually absent in the liquid. Moreover, the frequencies generally shift to lower values in going to the condensed phase. In addition, rather profound changes in intensity have been observed, particularly in the case of inactive fundamentals or with molecules that interact strongly. If one attempts to describe these spectral changes in quantitative terms, the situation is considerably less satisfactory. For example, the extent to which molecules rotate in the liquid, solution or solid state has not been clearly determined except for a few simple molecules [2-5-j. Recent studies have suggested considerable rotation in solution for several relatively complex molecules [6]. The question of the effect of condensation or dissolution on the frequency of absorption bands has received a considerable amount of attention [7]. Much of this work has been concerned with the effect of solvents on absorption frequencies. However, in spite of this large volume of experimental and theoretical investigations, it must be admitted that no adequate theory exists which is capable of quantitatively predicting these solvent-induced frequency shifts. Recently, attention has been drawn to the role of the oscillator itself in the [l] [2] [3] [4] [5] [6] [7]
G. HERZBERG, Infrared and Ramun Spectra p. 531f. Van Nostrand, New York (1945). J. C. MCLENNAN and J. H. MCLEOD, Nature 123, 160 (1929). E. CATALANO and D. E. MILLIGAN, J. Chem. Phys. 30,45 (1959). G. W. ROBINSON and M. MCCARTY, Jr., J. Chem. Phys. 30,999 (1959). M. F. CRAWFORD, H. L. WELSH and J. H. HaaRonn, Can. J. Phys. 80,81 (1952). W. J. JONES and N. SHEPPARD, Trans. Faraday Sot. 56, 625 (1960). For reviews of the literature on this subject see H. E. HALLAM, Conference onSpectroscopy. London, p. 1; Pergamon Press (1962); J. LASCOMBE Thesis, University ofBordeaux (1960). 1979
1980
R. E. KAQARISE
determination of these frequency shifts [8-91. On the basis of these and earlier studies, it seems clear that the specific structural properties of the oscillator itself are at least as important as those of the solvent in determining the magnitude and sign of the frequency shift. It would appear desirable, therefore, to be able to examine solvent-induced frequency shifts for all of the vibrational modes of a solute molecule. Experimentally, this is virtually impossible for solvent-induced shifts, because of interfering absorption by the solvent. Thus, one can usually study only a fraction of the solute’s absorption spectrum, namely those bands which are extremely intense or are favorable located with respect to solvent absorption bands. A second serious limitation to the use of solvents is the fact that recent unpublished studies at this Laboratory have shown the existence of specific solventsolute interactions even with non-polar, suppose~y inert solutions, such as carbon tetrachloride, carbon disulfide and benzene. Thus, the use of solvents to eliminate solute self-association may result in solvent-solute interactions of comparable or even greater magnitude. An obvious approach to the problems enoountered in solvent studies is to eliminate the solvent and study the frequency shifts which occur in going from the vapor state to the pure liquid. By this means one can perhaps establish some relationship between vapor-liquid frequency shifts and the nature of the molecular vibration. While some previous work has been conducted along these lines [l, 10, 111,no systematic or comprehensive investigation appears to have been conducted. The methyl halides provide an interesting group of compounds for studying vapor-liquid frequency shifts for several reasons. First, they provide a series of structurally similar molecules whose physical and chemical properties change in a monotonic manner. Thus, one can study the effect of the halogen substituents on the frequency shifts by observing trends in the series. Secondly, although the methyl halides are sufficiently complex to possess a variety of vibrational modes, their spectra are well understood and reliable assignments are available. Finally, the absolute intensities of the fundamental absorption bands have been measured [12] and normal coordinates calculated [13]. In addition to the methyl halides, several other relatively simple molecules were studied. These were methylene chloride, methylene bromide and methyl cyanide. An attempt was made to study methylene iodide, but the low vapor pressure of this material at room temperature restricted observation to a few very strong bands, and these fragmentary results will not be reported. EXPERIMENTAL Several spectrometers were used to obtain the spectra reported in this investigation. The choioe of the particular instrument to be used was dictated primarily [SJ G. L. CALDOWand H. W. THOPIPSON,Proo. Roy. Sot. A2S4, 1 (1960). [Q] C. HEALD and H. W. TXOBPPBON, Pmt. Boy. SOG.A%&89 (1962). [lo] E. K. PLYER randW. 8. BENEDICT,J. Bemamb. H&. Bar. ~~~~a~ 47, 202 (1951). 1111 W. K. GLASS and A. D. E. PULLIN, Trans. Fam%ay SOG.S&25 (1983). [12] A. D. DICKSON, I. M. MILLS and B. CRAWFORD,Jr., J. C&m. Phys. f%‘, 445 (1957). (131 W. T. KINU, I. M. MILLS and B. CRAWFORD,Jr., J. Chem. Phys. 27, 455 (1957).
Vapor-liquid frequency shifts in some substituted methanes
1981
by the wavelength region being studied. For the 2-15,~ region, a Perkin-Elmer Model 112G spectrophotometer equipped with a 75 lines/mm Bausch and Lomb grating and a KBr fore-prism was usually employed. However, because of interference from atmospheric water vapor, the 5-7,~ region was more conveniently studied with a Perkin-Elmer Model 21 instrument equipped with a CaF, prism. The spectral interval from 15 to 35 ,u was studied with a Perkin-Elmer Model 21 spectrophotometer equipped with a CsBr prism. Vapor-phase studies were carried out using conventional absorption cells having path lengths of 10 cm and 1 m. Liquids were studied in absorption cells In order to study methyl chloride and methyl having a variety of thicknesses. bromide as liquids, a modified version of the low-temperature cell described by LORD et al. [14] was used. The modified cell consists of a regular liquid absorption cell, having CsBr windows, contained in an evacuable Dewar flask. The cell is connected by means of a thermal conductor to a reservoir, which contains an appropriate coolant. The two filling ports of the sample cell are connected to external filling ports by means of small-bore polyethylene tubing (0.034 in. I.D.). These tubes permit the introduction of vapour into the cooled sample cell where it is liquified. By appropriate control of the pressure at the two filling ports, the cell can be uniformly filled with relative ease. The thermal conductivity of polyethylene is sufficiently low to prevent undue cooling at the external filling ports. The material used in this investigation were of c.p. grade purity or higher and were used without additional purification. The experimentally observed frequency shifts recorded in subsequent sections are differences between the frequency in the vapor phase and that in the pure liquid or solution state. In the vapor phase, the frequency of the band is based on the band origin, while the frequency in the condensed state measurements is determined by the band maximum. RESULTS AND DISCUSSION
Methyl halides Before considering the experimental data, it seems advisable to briefly discuss the symmetry and fundamental vibrational modes of the methyl halides. Belonging to the point group CSv, the methyl halides have six fundamentals, three totally symmetric ones (A,) and three doubly degenerate ones (E), all of which are both Raman and infrared active. The totally symmetric modes, vi, y2 and Ye, give rise to parallel infrared bands, while the degenerate modes yp, y6 and y6 give rise to perpendicular infrared bands. The molecules are symmetrical tops, the least moment of inertia being the three-fold axis of symmetry (the C-X axis). The form of the fundamental modes of vibration may be described approximately as follows: vi, symmetric C-H stretching; vz, sy mmetric CH, deformation; v,,C-X stretching; v4, asymmetric C-H stretching; vg, asymmetric CH, deformation; vs, H&!-X bending vibration. The results of the vapor-liquid frequency shifts for CH,Cl, CH,Br and CH,I are given in Table 1. Methyl fluoride was not studied due to the unavailability of a suitable sample. The compounds were also studied in carbon tetrachloride solutions. [la] R. C. LORD,R. MCDONALDand F. A. MILLER,J. Opt. Sot. Am. 42, 149 (1962).
R.E.
1982
KAUARISE
A number of the vapor state frequencies listed in Table 1 were measured and found to be in satisfactory agreement with the values listed by HERZBERCJ(Ref [l], p. 315) and the latter values were used for convenience. Table 1. Vapor to liquid and vapor to Ccl, solution frequency shifts for the methyl halides CH,Br
CH,Cl
CH,I
-
Mode %.sP)* cm-l
big,
Av(ccI,)
&V&P,
cm-’
Av,uw
Vlbl)
2966.2
-6.7
-9.9
2972
- 14.7
%W
1354.9
- 8.9
-7.6
1305.1
-11.4
v&d
732.1
-23.1
- 14.0
611
v4(4
3041.8
-6.0
-12.5
%,(4
1454.6
- 13.6
-11.6
vs(d
1015.0
+1.0
%G41)
2878.8
a
+1.3
-20.3
-21.1
- 77.6
-15.4
Av(cc14)
VW8P)
cm-l
Awq,
-12.8
2969.8
-19.1
-6.4
1261.6
- 14.5
- 17.5
- 10.3
632.8
-8.2
3065.9
-1.6
-6.8
3060.3
- 15.2
1445.3
- 16.8
- 12.6
1440.3
-15.3
952.0
+2.6
+3.1
880.1
$2.4
2861
-25.4
-20.5
-90.8
- 66.3
2861
Av(caq) -12.5 - 9.6 -3.7 - 9.4 - 12.3 +3.4
-28.6
- 22.0
-98.5
- 66.0
-
* Vapor phase frequencies according to Her&erg [l] p. 315. t Av = v(liquid) - v(vapor). $ B = Summation of shifts for individual modes.
Since the liquid state values for the chloride and bromide were measured at reduced temperatures, approximately - 78”C, the role of the temperature change on the absorption frequencies was of some concern. To evaluate the magnitude of this effect, liquid methyl iodide was observed at room temperature and at a The observed depression in temperature just above its melting point, -66°C. frequency at the lower temperature was about 2 cm-l and was essentially constant throughout the spectral region. In view of the constancy and smallness of this effect, no correction of data observed at reduced temperatures was made. If one examines the data contained in Table 1, certain generalizations are apparent. First, in any given compound, the magnitude of the vapor-liquid frequency shift is considerably different for the various normal modes of vibration. For methyl chloride, Av varies from +l*O cm-l for vg to -23-l cmfl for va, the C-Cl stretching mode. Moreover, this variation is independent of the species of vibration. Nor does there appear to be any correlaton between the nature of the vibrational mode and the magnitude of the frequency shift. Thus the C-H deformation modes exhibit shifts quite comparable to those of the C-H stretching modes. If one compares the frequency shifts for a given vibrational mode in the three methyl halides, certain well defined trends are found to exist. For example, the C-X stretching frequency, va shows a marked decrease in Av in going from CH,Cl to CHJ, whereas v1 and v4, the C-H stretching modes, exhibit an opposite trend. It is also interesting to note that vs has a greater frequency in the liquid state than in the vapor and is unique in this respect. The observed shifts in carbon tetrachloride solutions exhibit similar trends in
Vapor-liquid frequency shifts in some substituted methanes
1983
going from the chloride to the iodide or comparing the various modes of vibration. Moreover, the magnitudes of the solution shifts are comparable to these observed for the pure liquids, even though the latter are relatively polar materials. For example, the sum of the Av’s for CH,Cl is -77.6 cm-l for the liquid state and, -75.4 cm-l for the CC& solution. According to the Kirkwood-Bauer-Magat theory which is given by Av/v = C(E - 1)/(2-z + l), one would predict liquid state shifts about twice as large as the solution state shifts. This prediction is based on dielectric constant values of 12.6 and 2.238 for CH,Cl and Ccl,, respectively. From these data, one would conclude that the dielectric constant of the medium is not the sole or even principal factor in determining the magnitude of the vapor to liquid or solution state frequency shift. The interpretation of these data in a quantitative manner is hampered by the lack of a theory which considers the role of the solute in determining the magnitude of the shift. Most existing theories treat this contribution as a constant factor, a well-known example being the Kirkwood-Bauer-Magat theory previously mentioned. The more elaborate theories of BUCKINGHAM[ 151 and BENSON and DRICKAMER [16] treat frequency shifts in terms of properties of the absorbing solute molecules. However, both theories consider the solute to be a diatomic molecule, an assumption which is obviously not always valid. Moreover, these theories require a knowledge of such parameters as polarizability, change in dipole moment with internal co-ordinates, ionization potential, interatomic distances, etc. Such information is generally lacking for polyatomic molecules, particularly if one considers that solvent-solute interactions are strongly dependent upon the orientation of the interacting molecules, BENSON and DRICKAMER [16] have stated that their theory does not appear to have quantitative applicability because the observed shifts are differences of larger opposing effects which are described in terms of poorly known Thus it would appear that a quantitative prediction of frequency variables. shifts is not feasable at the present time. If one examines the theories mentioned above, it will be noted that the derivatives of the dipole moment with respect to the interatomic distance, a,u/aS, plays an important role in determining the magnitude and direction of the frequency shift. Absolute intensity measurements give the magnitude of (ap/&3) 2 but the sign is undetermined. The difficulty of making an unambiguous determination of sign in a polyatomic molecule is well-known. However, DICKSON et al. [ 121 have measured the absolute intensities of the fundamental modes of CH,Cl, CH,Br and CHJ and have determined the signs of the dipole moment derivatives; this determination is unambiguous for the E-class vibrations. It is of interest, therefore, to compare the observed frequency shifts with the corresponding dipole moment derivatives. These data are summarized in Table 2 and shown graphically in Fig. 1. An examination of the figure shows that there is a general correlation between the relative frequency shift Aviv and the sign and magnitude of the dipole moment derivative. These vibrational modes having large negative a,~/%’ values have correspondingly large, negative AVIVvalues. On 1151A. D. BUCPINUHAM, hoc. Roy. Sot. A248, 169 (1958); A255, 32 (1960). -161 A. M. BENSON, Jr. and H. G. DRICKAMER, J. Chem. Phya. 27, 1164 (1957). 2
1984
R. E. KAGARISE
other hand, modes having even positive frequency shifts. greater detail, it is immediately frequency shifts. In particular, pounds are considerably greater the
I
I 4.6
I
-20
-2.4
I -1.2
pusitive derivatives give rise to small negative or However, if one examines the data in somewhat apparent that other factors are contributing to the the ahifts for the ~~(a~) mode in the three comthan one would predict on the basis of this simple
I -OS
i -04
f
aplas
0
QDI l0.4
I
+I‘2
+0*8
Fig. 1 Table 2. Relative frequency shifts in methyl halides CH,CI Mode
--.
CH,Br
Au(liq)lv
was [=I @ahye/&
Wl)
-2C?P x 10-s
+ 0*543
-4.95
w&d
-6.37
-0.173
-8.73
-3l*@J
-2.313
v&a,)
va(4
- 1.97
$0*284
v,(e)
-9-35
- 0.323
Q&l
+ O-98
-+-0.269
Av(liq)/v
+/as
Av[liq)/v -6.43
-0.321
- 11.59
- 0*473
--28*64
- 1.451
- 15.39
- 0.646
-2~49
+0*21s
-4.97
+ 0,153
+2*73
-0.346 5” o-245
x 10-a
a/J/as
+a+34
-11.62
x 10-s
CrrI,
+ 0.396
- IO*62
- 0,333
+ 2.73
+ 0*277
relationship. Moreover, the line does not pass through the origin indicating the presence of other factors. Finally, this relationship ignores the role of the solute, whioh is certainly not constant for the three methyl halides. Jkfethyl cyanide Structurally, methyl cyanide (or acetonitrile) is quite similar to the methyl halides. They belong to the same symmetry point group, Csu, so that their vibrational spectra are very similar, Vibrational modes due to the methyl group are
Vapor-liquid
frequency shifts in some substituted
methanes
1985
essentially identical and a comparison of these frequencies is easily carried out. According to the selection rules, the vibrational spectrum of acetonitrile should consist of eight fundamentals, four totally symmetric A, modes and four doubly degenerated E modes. The form of these vibrations may be described approximately as follows: YI, symmetrio C-H stretching; vZ, symmetric CH, deformation; Q, C=N stretching; vp, G-C stretching; v~, asymmetric C-H stretching; Q, asymmetric CH, deformation; Y,, CH, rocking; v8, C-&N deformation. The observed spectra of liquid and gaseous acetonitrile, together with the frequency shifts Av = v (liquid) - Y (vapor), are listed in Table 3. The frequencies for the band origins in the vapor phase are those reported by PARKER and NIELSEN [17]. In addition, the vapor-liquid shifts observed for methyl chloride are listed for comparison purposes. In general, there is a reasonable correlation between the shifts in acetonitrile and those observed for the methyl halides. One notes, for example, that l,(e), the CH, deformation mode is characterized by a small shift in each case. The frequency of the C-C stretching mode, Y*, also changes very little in going from the gas to the liquid state. Perhaps the most outstanding shift listed in Table 3 is that exhibited Table 3. Vapor-liquid Mode
%l&PCC (cm-l)
shifts in acetonitrile
v1iq
(cm-‘)
Avlia
3178.2
3166.2
- 12.0
Me)
3009.1
3003.5
-5.6
-6-O
M-d
2954.2
2545.3
-8.9
-6.7
2628
2627-9
-0.1
%3 +
Y.qf
v4 (A,)
Yg m
va + v4 (A,)
2304
2294.6
-9.4
vs (ah
2266.7
2254.6
-12.1
2v, (A, + J-0
2081.9
2067.6
- 14.3
v&)
1454.2
1442
-12.2
v&J
1389
1375
-14
v,(e)
1040.8
1037.5
-3.3
Yk @I)
920.3
917
-3.3
2Q (A, + E)
‘113
747
+34 + 17.5
vsfd vs + Y? m
361.4
378.9
2417.3
2411.4
-23-I - 13.6 -8.9 + 1.0
-5.9
by vg, the C-CEH deformation mode. The band due to the fundamental is 17.5 cm-l higher in frequency in the liquid as compared to the vapor. Moreover, the second harmonic at 713 cm-l in the vapor occurs at 747 cm-l in the liquid, an increase of 34 cm+l. It is of further interest to note t*hat the shift in the first overtone is almost exactly double that observed in the fundamental. This behavior is to be expected according to many of the theories for solvent-induced frequency shifts [15]. The usual character of vayg, i.e. a positive frequency shift is also reflected in the combination band (vQ + Q) at 2628 cm-l, which occurs at the same frequency in both states. On the other hand, the shift in the harmonic of v,. is more than four times that observed for the fundamental. 2171 F. W. PARKER and A. H. NIELSEN, J. Mol. Spectroec. 1, 107 { 1957).
R. E. KACIARISE
1986
Methylene chloride and bromide The methylene halides represent a group of materials which are closely related to the methyl halides. The symmetry is, of course, reduced, the methylene compounds belonging to the C,, point group. This reduction in symmetry removes the degeneracy of the three E-type modes of the methyl halides and all nine fundamentals are non-degenerate for the methylene halides. The vibrations are divided into four species; four A, modes, two B, modes, two B, vibrations and a single A, vibration which is inactive in the infrared. The observed spectra in the vapor and liquid state of methylene chloride and bromide are given in Table 4. An examination of the observed frequency shifts shows a reasonable correlation between the two compounds. For example, the smaller shifts are observed for y7 and v8 in both cases. It is interesting to note that these are rocking modes which also gave small or even positive shifts in the methyl halides and in a~etonit~ile. These data likewise show that the magnitude of the vapor-liquid frequency shift is independent of the symmetry species to which the vibration belongs. It should be mentioned that v5 for both compounds and vs for methylene chloride were not observed in the vapor phase due to their weakness. The first vibration, V&is inactive by selection rules. Table 4. Vapor-liquid
shifts in methylene halides
CH,Cl,
CH,Br#
Mode VWPI (cm-~)
WQ (cm-‘)
2998.3
2987.9
1467.0 713.5 *
1421*7 705.2 *
Not observed
1156
Not observed
3065-P
~7 (b,)
898.5
vg @,I
1209~0
~a (W
760.7
890*2 1265 739.6
VWJP) (cm-“)
~w.u (em-l)
- 10.4
3014.3
2988.0
-26.3
-45.3 -8.3 -
1364 590.6 *
1390 578*5 *
+26 -12.1 -
-
Not observed
1090
-
-
3077.5
3om5
-11-4
-2-3
814.1
812.8
-4.0
1195.7
1193.6
- 1.3 -6.2
-21.1
646.9
631.7
- 14.2
* Not observtsble; beyond range of CsBr prism.
This investigation has shown that vapor to liquid state frequency shifts are rather strongly dependent upon the nature of the vibrational mode. Within any given molecule, one may have both negative and positive shifts, i.e. the frequency in the liquid (or solution) state may be either less or greater than the corresponding frequency observed for the vapor. The magnitude and sign of these shifts exhibit a more or less fixed pattern within a group of related compounds. The factor which appears to play a dominant role in determining the frequency shift is the derivative of the dipole moment with respect to the symmetry coordinate. Since the square of this derivative is proportional to the absolute intensity of
Vapor-liquid
frequency shifts in some substituted methanes
1987
the vibrational band, one would expect the stronger bands to exhibit greater frequency shifts in either a positive or negative sense. Since the range of shifts for the methyl halides is not particularly great it would be desirable to test this hypothesis in other compounds. Of particular interest would be a molecule whose dipole moment derivatives assume large negative and If one could firmly establish the validity of such a relationship, positive values. it would be particularly valuable in determining the sign of dipole moment derivatives.
Vapour-liquid frequency shifts in some substituted methanes R. E. KA~ARISE U.S. Naval Research Laboratory,
Washington
25, D.C.
(Received 12 June 1963) Abstract-The frequency shifts which occur in going from the vapor phase to the pure liquid have been observed for methyl chloride, methyl bromide, methyl iodide, methyl cyanide, methylene chloride and methylene bromide. In addition, vapor to carbon tetrachloride solution frequency shifts were observed for the three methyl halides. The observed shifts were independent of the symmetry of the vibrational mode and assume a wide range of values within an A comparison of shifts between related molecules shows well-defined individual molecule. patterns. For example, the ys(e) fundamental in the methyl halide exhibits a small positive shift in all three molecules, whereas ~~(a,), the C-X stretching mode, is characterized by rather large negative shifts. Finally, a good correlation between a,u/&S’,the dipole moment derivative with respect to symmetry coordinate, and the vapor-liquid frequency shifts is observed for the methyl halide fundamentals. INTRODUCTION THE qualitative nature of the changes which occur in infrared absorption bands in going from the vapor to the liquid state is reasonably well-established [l]. In particular, the rotational fine structure, if present in the vapour state spectrum, is usually absent in the liquid. Moreover, the frequencies generally shift to lower values in going to the condensed phase. In addition, rather profound changes in intensity have been observed, particularly in the case of inactive fundamentals or with molecules that interact strongly. If one attempts to describe these spectral changes in quantitative terms, the situation is considerably less satisfactory. For example, the extent to which molecules rotate in the liquid, solution or solid state has not been clearly determined except for a few simple molecules [2-5-j. Recent studies have suggested considerable rotation in solution for several relatively complex molecules [6]. The question of the effect of condensation or dissolution on the frequency of absorption bands has received a considerable amount of attention [7]. Much of this work has been concerned with the effect of solvents on absorption frequencies. However, in spite of this large volume of experimental and theoretical investigations, it must be admitted that no adequate theory exists which is capable of quantitatively predicting these solvent-induced frequency shifts. Recently, attention has been drawn to the role of the oscillator itself in the [l] [2] [3] [4] [5] [6] [7]
G. HERZBERG, Infrared and Ramun Spectra p. 531f. Van Nostrand, New York (1945). J. C. MCLENNAN and J. H. MCLEOD, Nature 123, 160 (1929). E. CATALANO and D. E. MILLIGAN, J. Chem. Phys. 30,45 (1959). G. W. ROBINSON and M. MCCARTY, Jr., J. Chem. Phys. 30,999 (1959). M. F. CRAWFORD, H. L. WELSH and J. H. HaaRonn, Can. J. Phys. 80,81 (1952). W. J. JONES and N. SHEPPARD, Trans. Faraday Sot. 56, 625 (1960). For reviews of the literature on this subject see H. E. HALLAM, Conference onSpectroscopy. London, p. 1; Pergamon Press (1962); J. LASCOMBE Thesis, University ofBordeaux (1960). 1979
1980
R. E. KAQARISE
determination of these frequency shifts [8-91. On the basis of these and earlier studies, it seems clear that the specific structural properties of the oscillator itself are at least as important as those of the solvent in determining the magnitude and sign of the frequency shift. It would appear desirable, therefore, to be able to examine solvent-induced frequency shifts for all of the vibrational modes of a solute molecule. Experimentally, this is virtually impossible for solvent-induced shifts, because of interfering absorption by the solvent. Thus, one can usually study only a fraction of the solute’s absorption spectrum, namely those bands which are extremely intense or are favorable located with respect to solvent absorption bands. A second serious limitation to the use of solvents is the fact that recent unpublished studies at this Laboratory have shown the existence of specific solventsolute interactions even with non-polar, suppose~y inert solutions, such as carbon tetrachloride, carbon disulfide and benzene. Thus, the use of solvents to eliminate solute self-association may result in solvent-solute interactions of comparable or even greater magnitude. An obvious approach to the problems enoountered in solvent studies is to eliminate the solvent and study the frequency shifts which occur in going from the vapor state to the pure liquid. By this means one can perhaps establish some relationship between vapor-liquid frequency shifts and the nature of the molecular vibration. While some previous work has been conducted along these lines [l, 10, 111,no systematic or comprehensive investigation appears to have been conducted. The methyl halides provide an interesting group of compounds for studying vapor-liquid frequency shifts for several reasons. First, they provide a series of structurally similar molecules whose physical and chemical properties change in a monotonic manner. Thus, one can study the effect of the halogen substituents on the frequency shifts by observing trends in the series. Secondly, although the methyl halides are sufficiently complex to possess a variety of vibrational modes, their spectra are well understood and reliable assignments are available. Finally, the absolute intensities of the fundamental absorption bands have been measured [12] and normal coordinates calculated [13]. In addition to the methyl halides, several other relatively simple molecules were studied. These were methylene chloride, methylene bromide and methyl cyanide. An attempt was made to study methylene iodide, but the low vapor pressure of this material at room temperature restricted observation to a few very strong bands, and these fragmentary results will not be reported. EXPERIMENTAL Several spectrometers were used to obtain the spectra reported in this investigation. The choioe of the particular instrument to be used was dictated primarily [SJ G. L. CALDOWand H. W. THOPIPSON,Proo. Roy. Sot. A2S4, 1 (1960). [Q] C. HEALD and H. W. TXOBPPBON, Pmt. Boy. SOG.A%&89 (1962). [lo] E. K. PLYER randW. 8. BENEDICT,J. Bemamb. H&. Bar. ~~~~a~ 47, 202 (1951). 1111 W. K. GLASS and A. D. E. PULLIN, Trans. Fam%ay SOG.S&25 (1983). [12] A. D. DICKSON, I. M. MILLS and B. CRAWFORD,Jr., J. C&m. Phys. f%‘, 445 (1957). (131 W. T. KINU, I. M. MILLS and B. CRAWFORD,Jr., J. Chem. Phys. 27, 455 (1957).
Vapor-liquid frequency shifts in some substituted methanes
1981
by the wavelength region being studied. For the 2-15,~ region, a Perkin-Elmer Model 112G spectrophotometer equipped with a 75 lines/mm Bausch and Lomb grating and a KBr fore-prism was usually employed. However, because of interference from atmospheric water vapor, the 5-7,~ region was more conveniently studied with a Perkin-Elmer Model 21 instrument equipped with a CaF, prism. The spectral interval from 15 to 35 ,u was studied with a Perkin-Elmer Model 21 spectrophotometer equipped with a CsBr prism. Vapor-phase studies were carried out using conventional absorption cells having path lengths of 10 cm and 1 m. Liquids were studied in absorption cells In order to study methyl chloride and methyl having a variety of thicknesses. bromide as liquids, a modified version of the low-temperature cell described by LORD et al. [14] was used. The modified cell consists of a regular liquid absorption cell, having CsBr windows, contained in an evacuable Dewar flask. The cell is connected by means of a thermal conductor to a reservoir, which contains an appropriate coolant. The two filling ports of the sample cell are connected to external filling ports by means of small-bore polyethylene tubing (0.034 in. I.D.). These tubes permit the introduction of vapour into the cooled sample cell where it is liquified. By appropriate control of the pressure at the two filling ports, the cell can be uniformly filled with relative ease. The thermal conductivity of polyethylene is sufficiently low to prevent undue cooling at the external filling ports. The material used in this investigation were of c.p. grade purity or higher and were used without additional purification. The experimentally observed frequency shifts recorded in subsequent sections are differences between the frequency in the vapor phase and that in the pure liquid or solution state. In the vapor phase, the frequency of the band is based on the band origin, while the frequency in the condensed state measurements is determined by the band maximum. RESULTS AND DISCUSSION
Methyl halides Before considering the experimental data, it seems advisable to briefly discuss the symmetry and fundamental vibrational modes of the methyl halides. Belonging to the point group CSv, the methyl halides have six fundamentals, three totally symmetric ones (A,) and three doubly degenerate ones (E), all of which are both Raman and infrared active. The totally symmetric modes, vi, y2 and Ye, give rise to parallel infrared bands, while the degenerate modes yp, y6 and y6 give rise to perpendicular infrared bands. The molecules are symmetrical tops, the least moment of inertia being the three-fold axis of symmetry (the C-X axis). The form of the fundamental modes of vibration may be described approximately as follows: vi, symmetric C-H stretching; vz, sy mmetric CH, deformation; v,,C-X stretching; v4, asymmetric C-H stretching; vg, asymmetric CH, deformation; vs, H&!-X bending vibration. The results of the vapor-liquid frequency shifts for CH,Cl, CH,Br and CH,I are given in Table 1. Methyl fluoride was not studied due to the unavailability of a suitable sample. The compounds were also studied in carbon tetrachloride solutions. [la] R. C. LORD,R. MCDONALDand F. A. MILLER,J. Opt. Sot. Am. 42, 149 (1962).
R.E.
1982
KAUARISE
A number of the vapor state frequencies listed in Table 1 were measured and found to be in satisfactory agreement with the values listed by HERZBERCJ(Ref [l], p. 315) and the latter values were used for convenience. Table 1. Vapor to liquid and vapor to Ccl, solution frequency shifts for the methyl halides CH,Br
CH,Cl
CH,I
-
Mode %.sP)* cm-l
big,
Av(ccI,)
&V&P,
cm-’
Av,uw
Vlbl)
2966.2
-6.7
-9.9
2972
- 14.7
%W
1354.9
- 8.9
-7.6
1305.1
-11.4
v&d
732.1
-23.1
- 14.0
611
v4(4
3041.8
-6.0
-12.5
%,(4
1454.6
- 13.6
-11.6
vs(d
1015.0
+1.0
%G41)
2878.8
a
+1.3
-20.3
-21.1
- 77.6
-15.4
Av(cc14)
VW8P)
cm-l
Awq,
-12.8
2969.8
-19.1
-6.4
1261.6
- 14.5
- 17.5
- 10.3
632.8
-8.2
3065.9
-1.6
-6.8
3060.3
- 15.2
1445.3
- 16.8
- 12.6
1440.3
-15.3
952.0
+2.6
+3.1
880.1
$2.4
2861
-25.4
-20.5
-90.8
- 66.3
2861
Av(caq) -12.5 - 9.6 -3.7 - 9.4 - 12.3 +3.4
-28.6
- 22.0
-98.5
- 66.0
-
* Vapor phase frequencies according to Her&erg [l] p. 315. t Av = v(liquid) - v(vapor). $ B = Summation of shifts for individual modes.
Since the liquid state values for the chloride and bromide were measured at reduced temperatures, approximately - 78”C, the role of the temperature change on the absorption frequencies was of some concern. To evaluate the magnitude of this effect, liquid methyl iodide was observed at room temperature and at a The observed depression in temperature just above its melting point, -66°C. frequency at the lower temperature was about 2 cm-l and was essentially constant throughout the spectral region. In view of the constancy and smallness of this effect, no correction of data observed at reduced temperatures was made. If one examines the data contained in Table 1, certain generalizations are apparent. First, in any given compound, the magnitude of the vapor-liquid frequency shift is considerably different for the various normal modes of vibration. For methyl chloride, Av varies from +l*O cm-l for vg to -23-l cmfl for va, the C-Cl stretching mode. Moreover, this variation is independent of the species of vibration. Nor does there appear to be any correlaton between the nature of the vibrational mode and the magnitude of the frequency shift. Thus the C-H deformation modes exhibit shifts quite comparable to those of the C-H stretching modes. If one compares the frequency shifts for a given vibrational mode in the three methyl halides, certain well defined trends are found to exist. For example, the C-X stretching frequency, va shows a marked decrease in Av in going from CH,Cl to CHJ, whereas v1 and v4, the C-H stretching modes, exhibit an opposite trend. It is also interesting to note that vs has a greater frequency in the liquid state than in the vapor and is unique in this respect. The observed shifts in carbon tetrachloride solutions exhibit similar trends in
Vapor-liquid frequency shifts in some substituted methanes
1983
going from the chloride to the iodide or comparing the various modes of vibration. Moreover, the magnitudes of the solution shifts are comparable to these observed for the pure liquids, even though the latter are relatively polar materials. For example, the sum of the Av’s for CH,Cl is -77.6 cm-l for the liquid state and, -75.4 cm-l for the CC& solution. According to the Kirkwood-Bauer-Magat theory which is given by Av/v = C(E - 1)/(2-z + l), one would predict liquid state shifts about twice as large as the solution state shifts. This prediction is based on dielectric constant values of 12.6 and 2.238 for CH,Cl and Ccl,, respectively. From these data, one would conclude that the dielectric constant of the medium is not the sole or even principal factor in determining the magnitude of the vapor to liquid or solution state frequency shift. The interpretation of these data in a quantitative manner is hampered by the lack of a theory which considers the role of the solute in determining the magnitude of the shift. Most existing theories treat this contribution as a constant factor, a well-known example being the Kirkwood-Bauer-Magat theory previously mentioned. The more elaborate theories of BUCKINGHAM[ 151 and BENSON and DRICKAMER [16] treat frequency shifts in terms of properties of the absorbing solute molecules. However, both theories consider the solute to be a diatomic molecule, an assumption which is obviously not always valid. Moreover, these theories require a knowledge of such parameters as polarizability, change in dipole moment with internal co-ordinates, ionization potential, interatomic distances, etc. Such information is generally lacking for polyatomic molecules, particularly if one considers that solvent-solute interactions are strongly dependent upon the orientation of the interacting molecules, BENSON and DRICKAMER [16] have stated that their theory does not appear to have quantitative applicability because the observed shifts are differences of larger opposing effects which are described in terms of poorly known Thus it would appear that a quantitative prediction of frequency variables. shifts is not feasable at the present time. If one examines the theories mentioned above, it will be noted that the derivatives of the dipole moment with respect to the interatomic distance, a,u/aS, plays an important role in determining the magnitude and direction of the frequency shift. Absolute intensity measurements give the magnitude of (ap/&3) 2 but the sign is undetermined. The difficulty of making an unambiguous determination of sign in a polyatomic molecule is well-known. However, DICKSON et al. [ 121 have measured the absolute intensities of the fundamental modes of CH,Cl, CH,Br and CHJ and have determined the signs of the dipole moment derivatives; this determination is unambiguous for the E-class vibrations. It is of interest, therefore, to compare the observed frequency shifts with the corresponding dipole moment derivatives. These data are summarized in Table 2 and shown graphically in Fig. 1. An examination of the figure shows that there is a general correlation between the relative frequency shift Aviv and the sign and magnitude of the dipole moment derivative. These vibrational modes having large negative a,~/%’ values have correspondingly large, negative AVIVvalues. On 1151A. D. BUCPINUHAM, hoc. Roy. Sot. A248, 169 (1958); A255, 32 (1960). -161 A. M. BENSON, Jr. and H. G. DRICKAMER, J. Chem. Phya. 27, 1164 (1957). 2
1984
R. E. KAGARISE
other hand, modes having even positive frequency shifts. greater detail, it is immediately frequency shifts. In particular, pounds are considerably greater the
I
I 4.6
I
-20
-2.4
I -1.2
pusitive derivatives give rise to small negative or However, if one examines the data in somewhat apparent that other factors are contributing to the the ahifts for the ~~(a~) mode in the three comthan one would predict on the basis of this simple
I -OS
i -04
f
aplas
0
QDI l0.4
I
+I‘2
+0*8
Fig. 1 Table 2. Relative frequency shifts in methyl halides CH,CI Mode
--.
CH,Br
Au(liq)lv
was [=I @ahye/&
Wl)
-2C?P x 10-s
+ 0*543
-4.95
w&d
-6.37
-0.173
-8.73
-3l*@J
-2.313
v&a,)
va(4
- 1.97
$0*284
v,(e)
-9-35
- 0.323
Q&l
+ O-98
-+-0.269
Av(liq)/v
+/as
Av[liq)/v -6.43
-0.321
- 11.59
- 0*473
--28*64
- 1.451
- 15.39
- 0.646
-2~49
+0*21s
-4.97
+ 0,153
+2*73
-0.346 5” o-245
x 10-a
a/J/as
+a+34
-11.62
x 10-s
CrrI,
+ 0.396
- IO*62
- 0,333
+ 2.73
+ 0*277
relationship. Moreover, the line does not pass through the origin indicating the presence of other factors. Finally, this relationship ignores the role of the solute, whioh is certainly not constant for the three methyl halides. Jkfethyl cyanide Structurally, methyl cyanide (or acetonitrile) is quite similar to the methyl halides. They belong to the same symmetry point group, Csu, so that their vibrational spectra are very similar, Vibrational modes due to the methyl group are
Vapor-liquid
frequency shifts in some substituted
methanes
1985
essentially identical and a comparison of these frequencies is easily carried out. According to the selection rules, the vibrational spectrum of acetonitrile should consist of eight fundamentals, four totally symmetric A, modes and four doubly degenerated E modes. The form of these vibrations may be described approximately as follows: YI, symmetrio C-H stretching; vZ, symmetric CH, deformation; Q, C=N stretching; vp, G-C stretching; v~, asymmetric C-H stretching; Q, asymmetric CH, deformation; Y,, CH, rocking; v8, C-&N deformation. The observed spectra of liquid and gaseous acetonitrile, together with the frequency shifts Av = v (liquid) - Y (vapor), are listed in Table 3. The frequencies for the band origins in the vapor phase are those reported by PARKER and NIELSEN [17]. In addition, the vapor-liquid shifts observed for methyl chloride are listed for comparison purposes. In general, there is a reasonable correlation between the shifts in acetonitrile and those observed for the methyl halides. One notes, for example, that l,(e), the CH, deformation mode is characterized by a small shift in each case. The frequency of the C-C stretching mode, Y*, also changes very little in going from the gas to the liquid state. Perhaps the most outstanding shift listed in Table 3 is that exhibited Table 3. Vapor-liquid Mode
%l&PCC (cm-l)
shifts in acetonitrile
v1iq
(cm-‘)
Avlia
3178.2
3166.2
- 12.0
Me)
3009.1
3003.5
-5.6
-6-O
M-d
2954.2
2545.3
-8.9
-6.7
2628
2627-9
-0.1
%3 +
Y.qf
v4 (A,)
Yg m
va + v4 (A,)
2304
2294.6
-9.4
vs (ah
2266.7
2254.6
-12.1
2v, (A, + J-0
2081.9
2067.6
- 14.3
v&)
1454.2
1442
-12.2
v&J
1389
1375
-14
v,(e)
1040.8
1037.5
-3.3
Yk @I)
920.3
917
-3.3
2Q (A, + E)
‘113
747
+34 + 17.5
vsfd vs + Y? m
361.4
378.9
2417.3
2411.4
-23-I - 13.6 -8.9 + 1.0
-5.9
by vg, the C-CEH deformation mode. The band due to the fundamental is 17.5 cm-l higher in frequency in the liquid as compared to the vapor. Moreover, the second harmonic at 713 cm-l in the vapor occurs at 747 cm-l in the liquid, an increase of 34 cm+l. It is of further interest to note t*hat the shift in the first overtone is almost exactly double that observed in the fundamental. This behavior is to be expected according to many of the theories for solvent-induced frequency shifts [15]. The usual character of vayg, i.e. a positive frequency shift is also reflected in the combination band (vQ + Q) at 2628 cm-l, which occurs at the same frequency in both states. On the other hand, the shift in the harmonic of v,. is more than four times that observed for the fundamental. 2171 F. W. PARKER and A. H. NIELSEN, J. Mol. Spectroec. 1, 107 { 1957).
R. E. KACIARISE
1986
Methylene chloride and bromide The methylene halides represent a group of materials which are closely related to the methyl halides. The symmetry is, of course, reduced, the methylene compounds belonging to the C,, point group. This reduction in symmetry removes the degeneracy of the three E-type modes of the methyl halides and all nine fundamentals are non-degenerate for the methylene halides. The vibrations are divided into four species; four A, modes, two B, modes, two B, vibrations and a single A, vibration which is inactive in the infrared. The observed spectra in the vapor and liquid state of methylene chloride and bromide are given in Table 4. An examination of the observed frequency shifts shows a reasonable correlation between the two compounds. For example, the smaller shifts are observed for y7 and v8 in both cases. It is interesting to note that these are rocking modes which also gave small or even positive shifts in the methyl halides and in a~etonit~ile. These data likewise show that the magnitude of the vapor-liquid frequency shift is independent of the symmetry species to which the vibration belongs. It should be mentioned that v5 for both compounds and vs for methylene chloride were not observed in the vapor phase due to their weakness. The first vibration, V&is inactive by selection rules. Table 4. Vapor-liquid
shifts in methylene halides
CH,Cl,
CH,Br#
Mode VWPI (cm-~)
WQ (cm-‘)
2998.3
2987.9
1467.0 713.5 *
1421*7 705.2 *
Not observed
1156
Not observed
3065-P
~7 (b,)
898.5
vg @,I
1209~0
~a (W
760.7
890*2 1265 739.6
VWJP) (cm-“)
~w.u (em-l)
- 10.4
3014.3
2988.0
-26.3
-45.3 -8.3 -
1364 590.6 *
1390 578*5 *
+26 -12.1 -
-
Not observed
1090
-
-
3077.5
3om5
-11-4
-2-3
814.1
812.8
-4.0
1195.7
1193.6
- 1.3 -6.2
-21.1
646.9
631.7
- 14.2
* Not observtsble; beyond range of CsBr prism.
This investigation has shown that vapor to liquid state frequency shifts are rather strongly dependent upon the nature of the vibrational mode. Within any given molecule, one may have both negative and positive shifts, i.e. the frequency in the liquid (or solution) state may be either less or greater than the corresponding frequency observed for the vapor. The magnitude and sign of these shifts exhibit a more or less fixed pattern within a group of related compounds. The factor which appears to play a dominant role in determining the frequency shift is the derivative of the dipole moment with respect to the symmetry coordinate. Since the square of this derivative is proportional to the absolute intensity of
Vapor-liquid
frequency shifts in some substituted methanes
1987
the vibrational band, one would expect the stronger bands to exhibit greater frequency shifts in either a positive or negative sense. Since the range of shifts for the methyl halides is not particularly great it would be desirable to test this hypothesis in other compounds. Of particular interest would be a molecule whose dipole moment derivatives assume large negative and If one could firmly establish the validity of such a relationship, positive values. it would be particularly valuable in determining the sign of dipole moment derivatives.