Study of the rotation-vibration band 2v4 of CH3F

RAUNO ANTTILA AND MIKKO H U H A N A ~ I Department of Physics, Uniuersity of Oulw, Oulw, Finland

Received May 27, 1968

The perpendicular component of the 2v4 band of CHjF around 6000 cm-I has been investigated. Besides the Q branches, R and P lines have also been measured and assigned to subbands for K < 4. Using several combination plots and ground-state constants from previous investigations, molecular constants in the excited state have been derived. The following results, among other things, were obtained: B' = 0.8560 f 0.0064 cm-l, B' - B = 0.0042 f 0.0004 cm-l, A' - A" - (B' - B") = 0.048 + 0.002 cm-l, and Cs4 = -0.170, which in turn gives C4 = 0.085 f 0.001.

Introduction Much work has been done on the infrared

spectra of methyl halides during the last few years. In the near infrared region for example, the bands of most compounds around 6000 cm-' have been investigated and also the corresponding bands of deuterated derivatives (cf. for example, Joffrin et al. 1966a, b ; Alamichel et al. 1967; Joffrin-Graffouillcre and Van Thanh 1967 ; Avia and Joffrin-Graffouillcre 1968). The near infrared spectrum of CH,F is, however, less completely known. The latest studies on the rotation-vibration bands of this molecule are those of Pickworth and Thompson (1954), Andersea et al. (1956), Smith and Mills (1963), and Jones et al. (1966), but only in the last one are there some results concerning the overtones and combinations in the near infrared region. Jones et al. have measured the positions of the Q branches in the perpendicular band v,. + v4 around 4050 cm-I and in the perpendicular component of the hybrid band 2v4 around 6000 cm"'. In the latter band there is a marked difference in the value of A' - A" - (B' - B"), compared with that expected from the data of the fundamental v4. The value obtained is also posi- tive, while the corresponding difference is negative for other methyl halides and for CH3B (Wiggins et al. 1953; Joffrin et al. 1966a, Joffrin- Grdfouill&re and Van Thanh 1967 ; Betrencourt- Stirnemann et al. 1967; Azria and Joffrin- Grdfouill&re 1968).

The molecular constants of CH3F in the ground state are k n o y from the microwave measure- ments of Thomas et al. (1 954) and from the infra- red investigatigns of Andersen et d. (referred to by Smith and Yills 1966).

The purpose~of this work was to study in detail the absorption around cm-I. Since vl

= 2965 cm-I and v, = 3009 cm-' (Pickworth and Thompson 1954), the first overtones and the combination of these fundamentals ought to exist near 6000 cm-l. The only band that could be identified in the recorded spectrum was the perpendicular component of 2v4. Besides the Q branches, the lines in various subbands also could be assigned and measured. The molecular constants were then derived using combination plots.

Experimental The sample gas was prepared by pyrolizing

tetramethyl ammonium fluoride, which was a product of Aldrich Chemical Co., Inc. Milwaukee, Wisconsin. The gas was in a 3-m gas cell. The pressure was about 100 mm Hg.

The measurements were performed with a high-resolution grating spectrometer described by Anttila (1967). A large echelle grating, 256 x 128 mm2, with 73.25 lines per millimeter was

-used, and the spectral slit width was 0.08 cm-l. The unwanted orders were sorted out with an interference filter. The detector was a lead sulphide cell.

The wave numbers were measured by inter- polating between emission calibration lines from noble gas spectral lamps. In the main part of the band the interpolation was performed with the aid of an interferometric wave-number scale. In the high-frequency end of the band, the scale was not applicable and so the accuracy is lower there. The lines have been measured from several different recordings.

Theoretical Methyl fluoride, as well as the other methyl

halides, is a symmetric prolate top and belongs to the C3, symmetry point group (Smith and Mills

Canadian Journal of Physia, 46,2025 (1968)

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2026 CANADIAN JOURNAL OF PHYSICS. VOL. 46, 1968

1963). Allowed frequencies for molecules such as this are derived from

[I] v = vo + F' - F",

where v, is the vibrational frequency and the rotational term value is (Herzberg 1960)

The single prime refers to the upper and the double prime to the lower state. c is the effective Coriolis coupling constant depending on the particular degenerate state. The other terms have their usual meaning.

The selection rules for the perpendicular bands are

When the upper state is degenerate and the lower one is nondegenerate, the above equations will give, for the Q branch (AJ = 0) frequencies of perpendicular bands (assuming Dj' = Dj", DJKt = DJK1', and DK = 0),

where the subband centers vosub are given by

[S] voSub = V, + A'(1 - 2 0 - B'

+ 2[A1(1 - c) - Bf]K

+ [(A' - B') - (A" - B")]K~.

where m = J + 1 for the R branches and m = - J for the P branches. The upper signs in [6] belong to the former and the lower signs to the latter branches.

When the centrifugal distortion terms are neglected in [6], the following combination relationships can be derived (Pickworth and Thompson 1954) :

[73 RRK-,(J- 1) - PPK+,(J+ 1) = 4(AU - A'< - B")K + 4BU(J + +),

C81 RRK-l(J)- PPK+l(J) = 4(AU - A'c - B")K + 4Bf(J + +),

[9] R ~ K ( J - 1) - 'P,(J + 1) = 4(Af - A'c - Bt)K + 4BU(J + +),

[lo1 RRILQ - PPK(J) = 4(A1 - A'c - B1)K + 4B1(J + +),

[1 I] R ~ K ( J - 1) + 'PP,(~) = 2[v0 + A'(1 - 2 0 - B']

+ 2[(A1 - A") - (B' - B")]K2

Results A fast run recording of the spectrum is given in

Fig. 1. The Q branches stand out strikingly and were easily detected at low pressure. The intensity variations strong, weak, weak, strong, etc. among the Q branches are clearly seen and the choice of which must be strong, is quite obvious. This assignment is in agreement with that of Jones et al. (1966).

In assigning the numerous observed lines to different subbands, there were two facts of importance. Hirst, on the basis of B" (Thomas eb al. 1954), the interval between adjacent lines was

In equations [ ~ I Y [ ~ I Y and L51 the upper signs and, known to. be about 1.7 cm- '. Secondly, there is respectively, the lower signs to each the restriction J 2 K. In particular, the starting other. lines of the RRK(J) branches, with J = K, are

The formula for the lines in the P and R easily seen when K 2 2 and thus they make branches (AJ = f 1) is (cf. Brown and Edwards unambiguous interpretations possible there. 1962) When K = 0, 1, or 2 even 'R,(J) and RpK(J)

m,K - 0 + (B' + B1')m Iines could be detected. The very weak first lines Kl vpSR - vsub

in the subbands with these lowest K values were, + (B' - ~ " ) r n ~ however, hard to find and the assignment in these

cases correspondingly remained equivocal to some extent.

- 2 r n ( ~ -+ 1)23 When K z 4, the 'P,(J) lines could neither be assigned unambiguously nor in accordance with - 4DJrn3, the constants derived from other subbands. The

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ANTTILA AND .HUKANANTTI: 2 ~ 4 BAND OF CH3F 2027

1 . 1 The perpendicular coinponent of the 2v4 band of CH3F.

'reason for the situation may be the disturbing effect of other, although obviously weak, bands at slightly lower wave numbers than 2v, (cf. Alamichel et al. 1967 and Jones et al. 1966).

Some observed lines have remained unex- plained. They are probably due to impurities or in some cases to other bands.

The measured wave numbers of the Q-branch maxima are given in Table I. In the same table the

TABLE 1

Positions of the Q-branch maxima and the subband origins in the perpendicular component of the 2v4 band

of CHgF

Subband Subband Observed origins origins Q-branch calculated calculated

Transition maxima from 141 from 65 3 K" + K' (cm- I) (cm - I) (cm -I)

subband origins are also given. These origins have been calculated by using two different methods. The second column gives the values calculated from the observed Q-branch maxima. The J values corresponding to these maxima can be evaluated according to the Honl-London theory (Herzberg 1960). Smith and Mills (1963) have calculated these values J, for CH,F and their values, together with the constants from Table 111, have been used to obtain the data in Table I from [4]. A least-squares fit of these wave numbers leads to the polynomial

The last column in Table I gives the values of sub- band origins calculated from equation [S]. The agreement between the last two columns is not very good. In many cases the maximum is evi- dently shifted by overlapping P or R lines from other subbands, but the differences may be partly due to resonance disturbances of other bands.

The measured P and R lines are collected in Table 11. In the analysis, use has been made of the previously determined ground-state molecular constants. According to Andersen et al. (as referred to by Smith and Mills 1963), A'" 5.081 cm-'. The microwave measurements of Thomas

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2028 CANADIAN JOURNAL OF PHYSICS. VOL. 46, 1968

et al. (1954) give B" = 0.851 793 cm-', Dj = These values, together with those obtained 1.98 x 10-~cm-', andDJK = 1.48 x 10-5crn-'. from eq. [I 11, give

To derive the molecular constants in the excited state from the measured frequencies. all A' - A'' - (B' - B") = 0.048 * 0.002 cm- ' the combinations [7]-[l l] have been calculated and plotted for K < 4. As an example of these graphs, the plot of the differences [9] versus J + 3 is given in Fig. 2. The slopes of the lines ought to be B" and the lines have been drawn using the above-mentioned literature value. The intercepa of the lines were then plotted against K to obtain A (1 - 6 ) - B" m e mean values from equations [7]-[lo] led to

The fit of the Q branches p2] leads to some- what different results:

A' - A" - (B' - B") = 0.047

The values derived here from the Q branches, like A' -A'& - B" 5.22 em-', A' -A" - (B ' - B") = 0.033 cm-' of Jones et al. (1966), can be considered less accurate than the above constants obtained from numerous subbands.

TABLE I1 Observed vacuum wave numbers in the perpendicular component of the 2v4 band of CH3F

K = O , A K = +1 K = l , A K = +1

AJ= +1 AJ= -1 AJ= +1 AJ= -1

obs. 1 0 3 ~ O ~ S . 1 0 2 ~ O ~ S . 1 0 2 ~ O ~ S . 1 0 2 ~ J (cm-l) (an-') (cm-l) (cm-l) (cm-l) (cm-l) (an-') (cm-l)

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ANTTILA AND HUHANANITI: Zv4 BAND OF CHIF 2029

TABLE I1 (concluded)

K = 2 , A K = -1 K = 3 , A K = + l , A K = -1

A J = + 1 A J = -1 A J = + 1 A J = -1

Qbs. 102A Qbs. 102A Obs. 102A Obs. 102A J (cm-I) (cm-') (cm-I) (cm-') (cm-') (cm-') (cm-I) (cm-I)

vrhe lines marked with an asterisk have been assigned more than once.

Those subbands where R as well as P lines The difference between the constants B gives could be found were used in addition to the = -0.0021 cm-' and this agrees well with combinations already mentioned to evaluate B' the result = -0.0019 cm-I from the and B' - B". The mean values of all the plots fundamental band v4 (Pickworth and Thompson together with the accepted value for B" give 1954).

B' - B" = 0.0042 f 0.0004 cm-I, Thus from the above results, A' = 5.133 + 0.002 cm-' (the possible error of A" is neglected)

B' = 0.8560. f 0.0004 cm-l. is obtained and then, according to equatiois ~ 1 3 1

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CANADIAN JOURNAL OF PHYSICS. VOL. 46, 1968

FIG. 2. Plots of RRK(J - 1) - PPI(J + 1) versus J.+ f for values K = 0 , I , 2, 3, and 4 in the perpendicular eomponmt of the Zv4 band of CH3F and a plot of the Intercepts aga~nst K.

the effective c of the 2v4 band will be - 0.170. This distortion has been taken into account by gives 5, = 0.085 + 0.o01, in fairly good agree- performing small corrections on the observed ment with the calculated value 0.088 and the data, according to the above-mentioned constants measured value 0.084 (from v, + v4) of Jones et of Thomas et al. (1954). The RRK(J) lines, with al. (1966). K = 5 6 , and 7, have also been regarded.

In the combination calculations the centrifugal The results have been collected in Table 111,

TABLE I11 Vibration-rotation constants derived from the perpendicular

component of the 2v4 band of CH3F together with the used literature values

B" = 0.851 793 c ~ - l (Thomas et al. 1954) B ' = 0.8560 em- B' - B" = 0.0042 em-', a4B - -0.0021 cm-' A' - A'" (I?' - B") = 0.048 cm-' A" = 5.081 em-: (Smith and Mills 1963) A' = 5.133 em- v0 + A ' (1 - 2c) - B' = 6 0 6 . 2 8 cm-l c44 = -0.170, c4 = 0.085 DJ = 1 . 9 8 ~ 10-: cm': (Thomas et al. 1954) DJK = 1.48 x 10- em- (Thomas et al. 1954)

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ANTTILA AND HUNANANTTI: 2v4 BAND OF CHIF 203 1

together with the used values from previous works.

The frequencies of all the observed lines were calculated-from equation [6] , using the above constants. The difference A = v,,, - vCalc are also to be seen in Table 11. The agreement is rather satisfactory; however, in many cases the deviations are far outside the possible experimental erron. It is thus believed ihat the band, especially at the low-frequency end, is perturbed, perhaps by v, + v, (cf. Jones et al. 1966). ,

Acknowledgment The authors wish to express their gratitude to

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AZRIA, R. and JOFFRIN-GRAEFOUILL~RE, C. 1968. Compt. Rend. 266B,75.

BBTRENCOTJRT-STIRNEMANN, C., ALAMICHEL, C., and GRANER, G. 1967. Compt. Rend. 265B, 549.

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19666. Can. J. Phys. 44, 2695. JOFFRW-GRAFFOUILL~RF~, C. and VAN THANH, N. 1967.

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