- Email: [email protected]
The vibrational spectra of alkyl selenocyanates
Spectrochimica Acta, Vol. 30A, pp. 337 'to 3)8. Pergamon Press 1974. Printed in Northern Ireland
The vibrational spectra o~ W. J.
FRANgr.rN,
alkyl selenocyanates
R. L. W~.~E~ and R, A. AShBy*
Chemistry Department, The New South Wales Institute of Technology, P.O. Box 123, Broadway, Sydney, N.S.W., Australia 2007 (Received 25 April 1973)
infrared spectra of methyl, ethyl, n-propyl, iso-propyl and n-butyl selenoeyanates have been examined in the region 4000-250 cm-1 as well as the Raman spectrum of the methyl compound. Vibrational analyses are presented for these compounds and there is evidence to suggest rotational isomerism about the C~--Se bond.
Abstract--The
INTRODUCTION IN GENERAL alkyi selenocyanates h a v e received c o m p a r a t i v e l y little a t t e n t i o n f r o m workers in t h e spectroscopic field. AYNSLEY et at. [1] h a v e r e p o r t e d t h e results o f infrared a n d R a m a n studies o f m e t h y l a n d p h e n y l selenoeyanate a n d it has b e e n r e p o r t e d [2, 3] t h a t organic selenocyanates can be distinguished f r o m t h e isomeric isoselenocyanates b y t h e i r respective a b s o r p t i o n characteristics in t h e region 2000-2200 cm -1. T h e present p a p e r describes t h e results o f a detailed s t u d y o f t h e spectra o f simple alkyl selenocyanates while a f u r t h e r p a p e r will be concerned with isoselenocyanates. As will be shown, t h e selenocyanates e x h i b i t certain features related t o r o t a t i o n a l isomerism similar t o those r e p o r t e d b y Hn~SCH~NN, et at. [4] for alkyl thiocyanates.
EXPERIMENTAL Methyl, ethyl, n-propyl, isopropyland n-butyl selenocyanates were prepared by mixing the appropriate alkyl iodide or bromide (0-28 mole) with a cold solution of potassium selenocyanate (0.28 mole) in methanol (60 ml). A n exothermic reaction commenced immediately on adding the iodide or bromide resultingin the deposition of the potassium halide. The mixture was gently refluxed for I0 rain after standing for 30 rain at room temperature. The solvent was removed by gentle warmth in a stream of nitrogen. The resultant slurry was steam distilled,giving the crude yellow selenoeyanate in 70-80 ~o yield. Vacuum distillationwith a nitrogen leak yielded a pale yellow product [5]. Infrared spectra were obtained with a Perkin-Elmer 521 grating spectrometer, calibration being carried out on each r u n using t h e sharp a b s o r p t i o n bands of gases [6]. T h e frequencies are corrected to v a c u u m a n d are considered t o be a c c u r a t e t o * To whom enquiries should be made regarding this paper. [1] E. E. AYNSLEY,N. N. GREEN~VOODand M. J. SP~AGUE,J. Chem. Soc. 2395 (1965). [2] T. TARANTELLIand D. LEONESI, A~n. Chim. (Rome) 58, 1113 (1963). [3] C. COLLARD-CHARONand M. RE~SON, Bull. Soc. Chim. Belges 72, 315 (1963). [4] R. P. I~RSCH~r~, R. N. K~ISELEY and V. A. FASSEL,Spectrochim. Acta f$O, 809 (1964). [5] W. J. FttA~T~rN and R. L. WER~rER, Tetrahedron Lett. 34, 3003 {1965). [6] H. W. THO~PSON(Editor) Tables of Wavenumbers for the,Calibration of InJrared Spectrometers, Butterworths, London (1961). 387
388
W.J.
I~RANKLIlq, R. L. WERNER and R. A. ASHBY
~-1 cm -1 for sharp bands. Low temperature spectra were obtained using a cell similar to that of RICHTEL and KLXPPMEIRER [7]. The vapour spectrum of methyl selenocyanate was obtained with a conventional 1 m gas cell heated to 65°C electrically. Some slight decomposition occurred during repeated scans but this was not a serious problem. B y repeated fractionation a nearly colourless sample of methyl selenocyanate was obtained and the R a m a n spectrum was recorded of this sample using a Hilger E612 spectrograph with a photoelectric scanning attachment. Band intensities were measured in terms of peak height relative to the carbon tetrachloride 458 cm -1 line recorded under identical conditions with correction for the response of the detector. The frequencies are considered to be accurate to ~=2 cm -1. Depolarisation ratios were measured b y the method of EDSALL and WILSON [8] and corrected by the procedure of KoNmsTEIN and BERNSTEIN [9]. RESULTS AND DISCUSSION OF SPECTRA The absorption frequencies together with proposed assignments are given in Tables 3-5 inclusive. In general it m a y be said that the low temperature spectra exhibited sharper bands compared to the liquid state as, for example, has been found for the alkyl halides. As the molecular weight of the compound increased the bands increased in width and there were fewer examples of crystal splitting. Before discussing the spectra of each molecule separately it is proposed to make a comparison between them for the purpose of distinguishing those features arising from the common functional group. Alkyl selenocyanates give rise to strong absorption in the region 2140-2160 cm -1, indeed, in the case of the five compounds studied the peak was measured at 2152 q- 2 cm -1. The integrated intensity of the band is close to 1 × 103 mole -1 1 cm -~ in all cases and the half-intensity bandwidth approximately 8 cm -1. The similar absorption bands in the case of alkyl thiocyanates lie at 2156 4- 3 cm -1 [4]. The location of these bands and the negligible frequency shift occurring on replacement of the sulphur with a selenium atom indicates an essentially uncoupled CN stretching mode. In view of this, the two remaining stretching modes of the C ~ S e - - C N chain ought properly to be sought in the region of C--Se stretching vibrations. HIRSC~MA~N et al. [4] have successfully interpreted the spectra of alkyl thiocyanates in terms of separate ~(C~--S) and ~(S--CN) vibrations and this receives indirect support from the microwave data on methyl thiocyanate [10] which show that the CSC angle is close to 100 °, implying no great interaction between the C ~ S and S - - C N arms. There are no structural data on alkyl selenocyanates b u t the vapour spectrum of methyl selenocyanate is very similar to methyl thiocyanate and there is, in general, a marked correspondence between the liquid selenocyanates and the appropriate thiocyanate. The premise is adopted, therefore, that a separate C~--Se [7] [8] [9] [10]
H. It. RICHELand F. It. KL_~PI~ErRE~, Appl. Spectry 18, 113 (1964). J. T. EDSALLand E. B. WILso~r, J. Chem. Phys. 8, 124 (1938). J. A. KONIGST~I~and I-I. J. BERNSTEI~,Spectrochim. Acta 18, 1249 (1962). S. NAGAKAWA,T. KOJIM.~, S. TXmCKAJKIand C. C. LI~, J. Mol. Spectry 14, 210 (1964).
The vibrational spectra of alkyl selenooyanates
389
and Se--CN stretching mode can be identified rather than asymmetric and symmetric modes involving all four atoms. The absorption of each of the compounds in the region 4000--250 cm -1 is shown in Figs. 1-4 for both liquid and solid states. I t would be anticipated t h a t v(C~-Se) should exhibit some dependence on the number of methyl groups attached to the C= atom as is the case with thiocyanates [4], thiols [11] and alkyl halides [12] and the frequencies 578, 545 and 530 cm -z for CHsSeCN, CzHsScCN and i-CsH~SeCN accord with this. I n Table 1 these values are compared with the corresponding C~--X frequencies found for alkyl chlorides, bromides, thiocyanates and dialkyl diselenides [13]. The last named is relevant since the long Se--Se bond in conjunction with a dihedral angle believed to be close to 90 ° has the effect of isolating the C--Se bonds, giving two mutually independent v(C---Se) vibrations of coincident frequency. As can be seen there is close agreement between C=--Br and C~---Se (-Se-) frequencies in the case of selenocyanates. Unfortunately, there appears to be no recorded value for di-isopropyl disulphide to permit a further comparison of disulphide with thiocyanate frequencies. I n further support of the assignments made, bands are found at 546 and 545 cm -z (the latter as a shoulder) in the spectra of n-propyl and n-butyl selenocyanate, these values agreeing closely with the value found for ethyl selenocyanate (secondary C= in each case). The possibility of hindered internal rotation about the C~---Cp bond should not be neglected. I n the case of n-propyl and n-butyl selenocyanate this will result in rotational isomers involving the position of the alkyl group with respect to the ~ . 0 5 0 mm 0'05,
3000
2000
Liquid
I000
cm "l
--V
Solid
Fig. 1. The infrared spectra of CHsSeCN (notations on spectra refer to sample thickness). [11] 1~. SHEPPARD, Trans. Faraday Soc. 46, 429 (1950). [12] J. J. SHaMAn, V. A. FeLT and S. KRIMm, Spectrochim. Acta 18, 1603 (1962). [13] G. BERGSON, Arkiv Kemi 13, 11 (1959).
390
W. J. FP.AN~IN, R. L. W~.~NF~ and R. A. ASHBY o'oqTmm
0"0'
o.opr
)7
0"0~
o-058mm i
2000
5000
Liquid
T I000
!
c m -~
Solid
Fig. 2. The infrared speetra of iso-CaHTSeCN (notations on spectra refer to sample thickness) \
~l
%oozmm
L..9~o7~ .----I--._
5000
2000
o.oorJ
Liquid
/
[000
(;m -I
i
Solid
Fig. 3. The infrared specbra of n-C3HTSeCN (notations on spectra refer to sample thickness).
C,--Se bond. I f the barriers to internal rotation are of the usual magnitude for C--C bonds, the isomerism ought to be detectable by the observation at ordinary temperatures of discrete absorption due to ~(C~---Se)for each isomer. As pointed out by HIRSCH~KANNet aZ. [4], two C,--C~ isomers are expected provided only staggered configurations are involved. The bands at 629 and 828 cm-1 in n-propyl and nbutyl selenocyanate are assigned to ~(C,--Se) in isomeric forms. The band in
The vibrational spectra of alkyl selenoeyanates
O.O~,Tmm
391
,D'O0? -
.
3000 -007turn
0-0072000 Ethyl
~ ~,.---v
o-o3s
I000
/0.184
cm-' ~"I O.IS4
N-but"yl
Fig. 4. The infrared spectra of CsHsSeCI~ and ~-C4HgSeCN (notations on spectra refer to sample thickness). Table 1. Correlation of C=--X frequencies (all frequencies given in em-1) Compound CHsX CHsCI-I~X (CHs)sCHX
X~-----C1 X ~ B r 712 656 615
594 560 536
X--S(SCN) 675 624 584
X~Se 570 536 517
(-Ses-)
X--Se
(SeCN)
578 545 530
n-propyl selenocyanate disappears completely on cooling to --180°0 and its assignment to a less stable isomer is convincing. The band in the spectrum of the n-butyl compound still tends to persist at low temperature, b u t there were indications that a glass rather than a crystalline film had formed and this m a y account for the inconsistent behaviour. The behaviour of the corresponding ~-butyl thiocyanate is not known. The second C~-Se stretching mode, i.e. v(Se--CN), is also subject to the effect of rotational isomerism. This bond, in all the compounds other than the methyl derivative, can "see" two isomeric configurations of the remainder of the molecule b y virtue of the hindered rotation about the C=--Se bond. A system of notation for such isomers is given b y H r R S C ~ N et al. and will be used here. Inspection of Figs. 1-4 shows t h a t after making the preceding assignments, the straight chain compounds (including methyl selenocyanate) each give rise to a band near 520 cm -1 which persists at --18000. The ethyl and higher ~-alkyl selenocyanates give, in addition, bands at about 565 cm -1 which appear as pronounced shoulders at room temperature and which have no counterparts in the spectra at low temperatures. The band at 520 cm -1 is assigned, therefore, in each case to v(Se--CN) in the more stable isomeric form, while the shoulder at 565 c m - x i s assigned to the less stable
392
W.J.
FRA-~KLI~, R. L. W~.RNER and R. A. Asma~-
form. I n iso-propyl selenocyanate ~(Se---CN) corresponding to the less stable isomer probably gives rise to the shoulder at 540 cm -1 which also disappears at low temperatures in the solid state spectrum. The corresponding band of the more stable isomer is taken to be the shoulder on the low frequency side of the band due to ~(C~--Se) at 530 cm -1. In the solid state the band appears distinctly at 512 cm -1. A distinction between thiocyanates and selenocyanates is apparent in the assignments above. In the thiocyanates ~(S--CN) for the less stable isomer occurs at frequencies consistently lower than its partner [4]. The reverse is true of ~(Se--CN) frequencies. Further, the frequencies of those bands which persist in the solid state crn-* 600 t
700 I
500 I
650 I
600 I
--CH3--
i
t
--C2H 5-
-n-C3H 7 -
-iso-C3H 7-
i Selenocyano'l-es
-n -C4H 9-
Thiocyonotes
Fig. 5. Correlation of C - - X stretching vibrations [solid lines connect frequencies associated with v(Ca--X ), and broken lines those associated with u(X---C(N)). ttorizontal bars denote bands which disappear at low temperatures (no data on n-C4HgSCN)].
are the same as that of ~(Se--CN) in the methyl compound which has an H configuration. I t may be, therefore, that this particular configuration is the more stable in the selenium compounds although, on more general arguments, the alternative C configurations would be preferred. The correlation of C--Se stretching frequencies in the various alkyl selenocyanates is illustrated in Fig. 5 which m a y be compared directly with that given for the C--S stretching frequencies of the thioeyanates [4]. The formal analogy is complete and it is probable the energy differences between the C~--Se rotational isomers is of the same magnitude as for the thiocyanates. Direct measurement of these differences by intensity measurements over a range of temperatures is scarcely possible owing to the extensive overlap of the v(Se--CN) and v(C~--Se) bands. The deformation vibrations of the CSeCN unit are readily identified by comparison of the spectra of the higher homologues with methyl selenoeyanate. Two of these, namely 6'(SeCN) (in-plane) and O"(SeCN) (out-of-plane) are assigned to the bands at 410 -~ 10 cm -1 (vw) and 363 ± 2 cm -1 (w) respectively. The choice is principally made in this order in view of the fact that the lower frequency band of the methyl selenoeyanate gives rise to a Raman depolarised line. Corresponding
The vibrational spectra of alkyl selenocyanate~
393
bands have been observed in phenyl selenocyanate [1] and there is a corresponding band at 357 cm -1 in selenocyanogen [14]. The third deformation mode 6(CSeC) can be assigned in methyl selcnocyanate to the polarised R a m a n line at 166 cm -1. The corresponding deformation vibration in dimethyl selenide falls at 236 cm -1 [15]. There is, therefore, a good correlation with the corresponding sulphur compounds since 6(CSC) in methyl thiocyanate has been assigned to a band at 190 cm -1 [16] while the deformation vibration in dimethyl sulphide falls at 285 cm -1 [15]. The higher alkyl selenocyanates were not examined below 250 cm -1 but it is anticipated that similar bands near 170 cm -1 would be encountered. Comments follow on particular members of the series: (a) Methyl selenocyanate The assignments proposed (Table 3) agree essentially with those given by AY•SLEY et al. [1] whose data are, however, less extensive. The comparison of the vibrational spectra of methyl thiocyanate and methyl selenocyanate convinces one, if this were needed, of the non-linear geometry of the heavy atom skeleton. The highest symmetry available then is point group Cs; the fifteen normal modes consisting of ten of species a' (v~s'CH3, vsCH a, ~CN, O~'CHa, 8sCHs, r'CH3, vSe--C, ~Se--CN, ~'SeCN and ~CSeC) and five of species a" (~as"CHs, (~as"CHs, r"CH s, 8"SeCN and vCHs). I t might be expected however, that, as in the case of methyl thiocyanate, the vibrations ~,'(CHs) and ~"(CHs) and the corresponding deformation pair would be coincident, at least in the condensed state spectra. On this basis the bands at 3040, 2946, 1420 and 1279 cm -1 are assigned to ~a~(CHs) (a' and a"), vs(CHs), ~as(CHs) (a' and a") and 0s(CHs), in t h a t order. The position is similar to that found for methyl thiocyanate [4, 17]. The two methyl rocking modes are almost coincident, giving rise, in the liquid spectrum to a strong absorption at 929 cm -1 and a barely defined shoulder at 920 cm -1. I n the solid state spectrum at low temperatures the strong band was found at 935 cm -1 while the weaker band, now quite distinct, occurred at 925 cm -1. Both bands exhibited crystal splitting. The methyl torsional mode probably lies near 100 cm -1 by analogy with the corresponding assignment for methyl thiocyanate [18]. The band at 166 cm -1, assigned to 8(CSeC), clearly lies at too high a frequency and is, in any case, polarised in the R a m a n effect. The infrared spectra of methyl thiocyanate and methyl selenocyanate in the vapour state are similar in m a n y respects. No detailed illustrations of the vapour spectrum of methyl thiocyanate have appeared in the literature although it has been described [4]. Some band contours for the thiocyanate, obtained during the course of this work are reproduced in Fig. 6 together with contours for methyl selenocyanate. Similarities in bands which correspond in the spectra of the two compounds suggests t h a t the thiocyanate and selenocyanate have similar structures. On the [14] E. E. AYNSLEY,1~. N. GREEN'WOODand M. J. SPRA(~UE,J. Chem. Soc. 704 (1964). [15] S. SIEBEI~T,Z. AlrbO~'g.AUgem. Chem. ~71, 65 (1951). [16] F. A. MTT.T.V.Rand W. B. WHITE, Z. Eldctrochem. 84, 701 (1960). [17] R. VOGEL-HOGLER,Aeta Phys. Austriaca 1, 311 (1948). [18] W. G. FATELEYand F. A. MZLLER,Speetrochim. Acta 17, 857 (1961).
394
W.J.
FRANKLIN, R. L. WERNER and R. A. ASHBY C m -I
3050 r
2950 T
T
2200 r
2150 1
6
.o do
Fig. 6. The infrared spectrum of CHsSeCN vapour [path length, 1 m; 80°C (inserts -CH.qSCN): spectral slit width 1 cm-1].
other hand, if one assumes that the CSeC angle is about 100 °, close to the value of 99052 ' appropriate to the CSC angle of methyl thiocyanate [10] then it is true that the b a n d contours of both compounds do not consistently reflect the molecular geometry. The vapour contours predicted are essentially those of a B-type asymmetric rotor in the case of ~(CH3) and ~ ( C H 3 ) ; an A-type band for ~(CN) ; mixed A- and C-type bands for coincident ~as(CH3) and ~(CH3) pairs of vibrational modes and A- and U- type bands respectively for the a' and a" rocking modes. The contours observed for the ~s(CHs) and Os(CH3) bands do not match these predictions although the other bands could be interpreted as conforming to the predictions made. I n actual fact, the bands due to ~s(CH3) and O~(CHs) suggest structures approaching closely tJhat expected of the true symmetric top and the interpretation of these contours for U, type molecules is clearly indicated. The contour of the band(s) in the region of the rocking vibrations in the vapour spectrum can be interpreted as a near superposition of a higher frequency A-type band and a lower frequency U-type band of which only the characteristically strong central branch is evident. Thus the higher frequency is taken to be the in-plane (a') rocking mode. Although it is not possible to draw firm conclusions as to the details of bonding in methyl selenocyanate from the vapour spectrum, there is reason to doubt that there is as much - S e + - - C - - N - back bonding character as there is -S+~C~-~N - in methyl thiocyanate [12]. Confirmation of this statement comes from a comparison of the ~(C--X) and • (CN) frequency assignments of these and related compounds (see Table 2). The ~(S--CN) frequencies fall in the same range as ~(S--CHs) frequencies whereas ~(Se--CN) falls at distinctly lower frequencies than ~(Se--CH3) and the presence of considerable S ~ C character in sulphur dicyanide is definitely established
The vibrational spectra of alkyl selenoeyanat~s
395
Table 2. Comparison of ~(C---X) and ~(CN) assignments for related compounds Compound
~(C--S)
CHsSCN
699 (S---CN)
675 ( c ~ - - s ) 741, 691 689 684, 665* 673, 667
CHa-~S--CH s CHs--(S)s--CH s S(CN)s (SCN)s~
v(CN)
Ref.
2158
4
Compound
~(C--So)
CHsSoCN
521 ( S e - - C l q )
---
-
2183, 2179 2171
v(CN)
Ref.
2152
576 (C~---Se) 19 19 14 20
CHs--Se---CH s CHs--(Se)s--CH s Se(CN) s (SeCN) s
603, 568 570 516(2)~" 521(2)
2171 2152
15 13 14 14
* C o n f i r m e d in R e f . [20]. t A s s i g n m e n t t o 608 (w) a n d 516 c m -1 p a i r in [14] s e e m s u n a c c e p t a b l e .
Table 3. Frequencies [cm-l(i.r.)] and assignments for CHsSeCN Liquid
Solid (--180°C)
3957 3862 3624 3520 3432 3039 2946 2820
3958
2672 2545
2679-2671~ 2559
2512 2329
2534-2526t 2323
3044 2953 2821
2198 2191 2152 2153 2104 2101 1850 ~ 1 8 5 0
Assignment
Liquid
3039 ~- 935* ~ 3964 2946 -~- 295* ~ 3871 3039 -~- 578 ~ 3617 2946 -~ 578 ~ 3524 2152 -~- 1278 ~ 3430 ~'as CHs(2) R a . depol? Vs C H s - - R a . pol. 1420 -~- 1420 ~ 2840 2946--r CH s (~125) 2152 -~- 521 ~ 2673 ~ 2 1 5 2 -~- 401 ~ 2553 L1278 -~- 1278 ~ 2556 2152 -~ 365 ~ 2517 2152 ~- 156 ~ 2318 1420 -~- 925* ~ 2345 1278 -~ 925* ~ 2203 V C N - - R a . pol. ? 925* -~ 925* ~ 1850 1278 -}- 578 ~ 1856
1721 1616 1501
Solid ( - - 180°C)
1420 1341
1423
1278 1146 1070 929 920 811 728 578 521 401 365 1665 100§
1278 1153 946-926 822 ~750 585-580~ 518 401 371
Assignment ? ? 925* -~- 578 ~ 1503 1420 -~- ~- C H s ( ~ 8 0 ) (~as C H s ( 2 ) - - R a - depol. 925* -~- 401 ~ 1326 1420-~ CH 3 (~80) ~ C H s - - R a . pol. 578 -~- 578 ~ 1156 925* ~- 166 ~ 1091 {r' C H s I.r" C H s 401 -~ 410 ~ 802 465 ~- 365 ~ 730. ~ S e - C H a - - R a . pol. ~ S e - C N - - R a . pol. ~ S o C N - - R a . pol. ~" S o C N - - R a . pol. ~ C S o C - - R a . pol. ~" C H s
* Average rocking frequencies. ' D e n o t e s i n - p l a n e ; " o u t o f . p l a n e in r e s p e c t o f Us s y m m e t r y . t C r y s t a l field s p l i t t i n g . :~ R a m a n f r e q u e n c y . § Postulated value.
[21]. Complementary behaviour of the ~(CN) frequencies is not clearly evident; nevertheless the integrated intensities of the ~(CN) bands of selenocyanates are only about half t h a t for thiocyanates suggesting a closer approximation to a true -C~____N configuration in the former. (b) Ethyl and i-propyl selenocyanate Previous analyses of the vibrations of ethyl and iso-propyl groups [4, 22] in conjunction with the preceding discussion on the vibrations of the CSeCN group make it possible to give nearly complete assignments for these two compounds. Those proposed in Table 4 are based on an assumed Cs s y m m e t r y for the molecules; the maximum possible in either case. The low s y m m e t r y results in m a n y equivocal [19] [20] [21] [22]
D. W. SCOTTand J. P. McC~-~LOUGH,J. Am. Chem. Soc. 8 0 , 3554 (1958). M. J. NELSO~and A. I). E. PI~-~LI~,J. Chem. Soc. 604 (1960). L. PIERCE, R. NELSO~and C. THOMAS,J. Chem. Phys. 43, 3423 (1965). N. SHEPP~D, J. Chem. Phys. 17, 79 (1949) and Trans. Faraday Soc. 43, 533 (1950).
396
W.J.
FRA~r~L~, R. L. W~R~E~ and R. A. ASHBY
Table 4. Frequencies [em-~(i.r.)] and assignments for CsHsSeC~Tand iso--CaH:SeCN CsHsSeCN Liquid 365 406 522 645 ~565 754 961 1017 1054 1092" 1237 1382 1430 1449§ 1500
Solid ( - - 180°C) 374 411 517 522 750 960 1016 1054 1092 1234 1380 1433 1457 1509
1621" 1777 1917 2102 2153 2188 2330 2426 2460 2509 2549 2672 2738 2824 2871 2928 2974** 3000 3127 3252 3384 3523 3620
2102 2154 2188
2837 2870 2933 2970 3006
Assignment (~" SeCN (~P SeCN v Se--Ol~ v C~--So v Se---CNt C H s rock v C---C(and r" CHs) CH s twist r' C H s 545 --~ 545 ~ 1090 CH s wag ~s CHs CH s bend (~as CHs(2) 754 -}- 754 ~ 1508 545 -~ 961 ~ 1506 1054 -~- 565 ~ 1619 t 754 -~ 1017 ~ 1771 545 -~- 1237 ~ 1782 961 -~ 961 ~ 1922 1054 -~ 1054 --~ 2108 v 01~ 754 -~- 1430 ~-~ 2184 961 -~- 1382 ~ 2343 1054 -~ 1382 ~ 2436 1017 -~ 1449 = 2466 1054 ~- 1449 ---~ 2503 406 -[- 2153 ~-- 2559 522 -}- 2153 ~ 2675 ~ 1382 -~ 1449 ~ 2831 Vs C H s, vs CHs vas C H s vas CHs(2) ? ? 2871 ~- 365 ~ 3236 r I 5 3 -~- 1237 ~ 3390 2871 -}- 522 ~ 3393 1382 ~- 2153 ~ 3535 2671 ~- 754 ~ 3625
Liquid
A" A' A' A" A" A' A" A' .4' A' A' A'A"
A'
363 419 444 512" 530 ,~540 628~ 727 770 823 877 933 950 1042 1117 1157 1219]] 12311[ 1282 1217 1373 1389 1458 1488" 1685
iso--CaH~SeCN Solid ( - - 180°C) Assignment 369 399 445 512 535
737 765 835 884 929 947 1041 1120 1161 1229 1276 1305 1367 1389 1440¶ 1488
1750
A'A' A" A'A"
1835 1910 2100 2152 2247
2099 2150
2302 2387
2420 2506 2538 2608 2676 2732 2767 2810 2869 2897 2928 2983 3200 3230 3371
2866 2888 2924 2969~
(~" SeCN (~' SeCN ~ CCC? v Se---CI~ v Ca---So ~ So---CN~ ? 363 -~- 363 ~ 726 363 -~- 419 ~ 782
A" A' A" A' .4'
v ' C-C .4' r" C H s A" r" CH s A" r' CH s A' v" C - - C A" r ' CH s A' ~ GH A' ($ C H A" 933 -~ 363 ~ 1296 877 ~- 444 ~ 1321 950 -t- 363 ~ 1313 (~s CHs A" (~s CHs A' ~as CHs(4) A ' ( 2 ) , A"(2) (~as CHs -~- lattice v i b r a t i o n 1317 ~- 363 ~-- 1680 1157 -~- 530 ~- 1687 877 -~ 877 ---- 1754 1389 -}- 363 ----- 1752 877 ~- 9 5 0 - ~ 1827 1373 -{- 530 = 1903 877 -I- 1231 ~ 2108 950 -~- 1157 ~ 2107 v CN A' 1317 -{- 933 ~ 2260 877 -{- 1373 ~ 2250 1157 -~- 1157 ~ 2314 1231 -[- 1157 ~ 2388 1440 -}- 950 ~ 2390 1042 -}- 1373 ~ 2415 2153 ~ 363 ~ 2516 1117 ~ 1389 ~ 2506 1157 -[- 1373 ~ 2530 1377 -.[- 1231 ~ 2608 2152 --~ 530 ~ 2682 1231 -~- 1440 ~ 2671 1373 -{- 1373 ~ 2746 1373 ~ 1389 ~ 2762 1440 -~- 1370 --~ 2813 vs CHs(2) A'A" 1440 -~- 1440 = 2880 1440 -~ 1458 ~ 2898 Vas CHs(2) A"A" ~as CHs(2) A' 2152 -{- 1042 ~ 3194 363 -~ 2869 ~ 3232 2152 ~- 1231 ~ 3383
* Solid s t a t e f r e q u e n c y only. t Less stable H g (~) isomer. § B a n d s a t (1457, 1455) a n d 1443 in solid. ** 4 b a n d s in solid. il Single b a n d in solid. ¶ F u r t h e r split in solid. ~ t B a n d s a t 2986, 2963, 2945 in solid. V e r y w e a k indeed, n o t to be confused w i t h Ca~-Se b a n d a t 628 e m -1 in t h e s t r a i g h t chain c o m p o u n d s - could in fact signify t h e presence of t h e n - p r o p y l isomer as a n i m p u r i t y .
The vibrational spectra of alkyl selenocyanates
397
combination and overtone assignments. The modes not assigned are those described as ~(CCSe) (2 in either molecule), ~(CSeC), ,(CHs) (one in the ethyl and three in the iso-propyl compound) and ~(C--H) in /8o-propyl selenocyanate. Apart from • (C--H) which is expected to be weak and obscured by ~(CHs) and ~(CH~) the modes not assigned lie below the frequency range investigated and it is noted t h a t one of the 5(CCSe) modes (the out-of-plane one) must be equivalent to a turning movement about the C,---Se bond, i.e. to the motion by which isomerisation occurs. There is some inconsistency in the CH~ bending assignment for ethyl selenocyanate. Applying the correlation between CH z deformation frequencies of XCH~Y molecules with the electronegativity product, ~(X)~(Y), reported by JoN~s and THOMAS [23], to ethyl thiocyanate and ethyl selenocyanate, the CH~ twisting, wagging and rocking frequencies of the former [4] are about 1.03 times those as assigned for the latter but the ~(CHz) frequencies are virtually the same. I f ~(CH~) in ethyl selenocyanate is taken to be coincident with ~(CHs) at 1382 cm -1, a ratio of 1.032 is obtained, but concomitant assignment of the 1430 cm -1 band to one of two possible binary combinations is less t h a n satisfactory because of its considerable intensity. I n the assignments for iso-propyl selenocyanate, the a' r(CHs) and ~(C--C) assignments are revised compared to those given by SHEPPARD following the precedent set in [4]. The two ~(CH) modes are assigned to the strong doublet at 1231, 1219 cm-L There seems no alternative, since to assign one of the 5(CH) vibrations to the very weak band at 1317 cm -1 leaves the doubling of the strong band unexplained. HIRSCHWA~I~ et al. [4] have assigned the ~(CH) modes in iso-propyl thiocyanate to 1318 (w) and 1247 (s) cm -1, and exception must be taken to this in view of the results on iso-propyl selenocyanate. The illustration of their spectrum in [24] shows t h a t the 1318 cm -1 band is very weak indeed, while the 1247 cm -1 band has a shoulder at 1258 cm -1, with two strong bands at 1247 and 1258 cm -1 appearing in the solid spectrum. I t is thus tempting to reassign the 1318 cm -1 band to a combination.
(C) n-Prowl and n-butyl selenocyanate8 Fundamental assignments (Table 5) have also been attempted for n-propyl and n-butyl selenocyanate based as far as possible on the work of SHEPPAI~D and SIMPSON on n-alkanes [25]. Those in the 1350-700 cm -1 region rest on schematic descriptions of the normal modes which, at best, can be only approximately correct, and combinations and overtones of the low frequency vibrations m a y also complicate the issue by virtue of Fermi resonance interaction. I n the n-butyl compound there is the added complication of rotational isomerism about the C~--Cp bond. The low temperature solid spectrum showed no marked differences from the liquid spectrum, which, as noted before, could be attributed to the formation of a glass. Those bands (above 720 cm -1) in n-propyl and n-butyl selenocyanate which have [23] R. G. JO~rESand W. J. ORVILLE-THOMAS,~_~ctrochi~. Acta 20, 291 (1964). [24] R. P. H I ~ S C H ~ , Ph.D. Thesis, Iowa State University of Science and Technology, 1963, Diss. Abs. 64-3875. [25] 1~. SBEEPPA.RDand D. M. SIMPSON,Quart. Rev. (London) 7, 19 (1953).
398
W.J.
F R A n K L I n , R . L . WERNER a n d R . A. ASHBY
T a b l e 5. F r e q u e n e i e s [ c m - Z ( i . r . ) ] a n d a s s i g n m e n t s f o r n - C a H ? S e C N a n d n - C a H 9 S e C N n-CaH~SeCI~ Solid (-- 1 8 0 ° C )
Assignment
Liquid
363
372
~ SoCN
413
413
~' SeCN (and ~ CCCD Se--CN v C~--Se So---CN* Ca---Se*
362 38O 400 468 520 545 ~565 628 725~ 738~ 776| 786] 868, 907
Liquid
521 546 ~565 629 727 771 825?
514 548
778-771 828
r CH 2 modes
884 896
886
Ys C--C--C Ys C--C--C*
1032 1039 1076 1093
1027 1041 1078 1090
tw -CH2--Se r CH 8 r CH a ~as C--C---C
1217
1224
wag -CHf--Se
1282
1281-1278
tw -CH~-
1342
1340
wag -CH z-
1383 1420 1443 1461 2152 2959 2876 2937 2968
1380 1422 1449 1469-1454 2155 2860 2872 2940-2937 2976-2967
(~s CHa (~ -CH~--Se (~ -CH~~as CHa (2) ~ CN ~s CH~ (2) vs CH a ~asCH~ (2) Vas CH8 (2)
n-C~HsSeCN Solid (--180°C) 368 372
~ SeCN ~ CCG (~ SoCN ~ CCC 3' So---CN ~ C~--Se~ ~ Se---CN*
514 547 629 724~ 737~ 771| 748.' 865, 893, 903
v C~--Se~ r CH 2 modes v C--C--C--C*
968, 995 965, 994 1009~ 1013 10491 1047 1062 1059 1099 1098 11655 ~ 1165, 1149~
12095 l !
1261,~ 1290, 1297~}
|
1344,1351 1359, 1369 1382 1421 1439 1467 2152 2867 2877 2937 2963
| J
Assignment
v C--C--C--Ct r CH a and tw -CH~--So r CH a v C--C--C--C
1204 l 1295 1259 1304 1357 1360 1378 1419 1435 1465 2150 2871
|~ | |
tw and wag CHI modes
}
2932 2958
~s CH s (~ -CH~--So (~ -CH~(~as CH~ (2) v CN Vs CH~ vs CH 8 ~as CH~ ~as CHa (2)
* Less stable rotational isomer. ~f In different rotational isomers. 5 Probably more closely associated with -CH2--Se, by comparison with n~C4HgSCI~T. • Denotes out.of-plane; ~ in-plane with respect to CSeCN group.
appreciably different frequencies when compared to the otherwise very similar thiocyanate spectra [4] are taken to be associated mainly with the CH 2 group adjacent to the selenium atom. The O(CH2) mode again appears not to shift on exchanging the SCN group for the SeCN group.
Acknowledgements--One o f
u s (W. J . F . ) is i n d e b t e d t o t h e C . S . I . R . O . a n d t o t h e U n i v e r s i t y o f New South Wales who, in turn, granted him post-g ra dua te s tude nts hips without whic h he would have been unable to participate in the work re porte d herein.
The vibrational spectra o~ W. J.
FRANgr.rN,
alkyl selenocyanates
R. L. W~.~E~ and R, A. AShBy*
Chemistry Department, The New South Wales Institute of Technology, P.O. Box 123, Broadway, Sydney, N.S.W., Australia 2007 (Received 25 April 1973)
infrared spectra of methyl, ethyl, n-propyl, iso-propyl and n-butyl selenoeyanates have been examined in the region 4000-250 cm-1 as well as the Raman spectrum of the methyl compound. Vibrational analyses are presented for these compounds and there is evidence to suggest rotational isomerism about the C~--Se bond.
Abstract--The
INTRODUCTION IN GENERAL alkyi selenocyanates h a v e received c o m p a r a t i v e l y little a t t e n t i o n f r o m workers in t h e spectroscopic field. AYNSLEY et at. [1] h a v e r e p o r t e d t h e results o f infrared a n d R a m a n studies o f m e t h y l a n d p h e n y l selenoeyanate a n d it has b e e n r e p o r t e d [2, 3] t h a t organic selenocyanates can be distinguished f r o m t h e isomeric isoselenocyanates b y t h e i r respective a b s o r p t i o n characteristics in t h e region 2000-2200 cm -1. T h e present p a p e r describes t h e results o f a detailed s t u d y o f t h e spectra o f simple alkyl selenocyanates while a f u r t h e r p a p e r will be concerned with isoselenocyanates. As will be shown, t h e selenocyanates e x h i b i t certain features related t o r o t a t i o n a l isomerism similar t o those r e p o r t e d b y Hn~SCH~NN, et at. [4] for alkyl thiocyanates.
EXPERIMENTAL Methyl, ethyl, n-propyl, isopropyland n-butyl selenocyanates were prepared by mixing the appropriate alkyl iodide or bromide (0-28 mole) with a cold solution of potassium selenocyanate (0.28 mole) in methanol (60 ml). A n exothermic reaction commenced immediately on adding the iodide or bromide resultingin the deposition of the potassium halide. The mixture was gently refluxed for I0 rain after standing for 30 rain at room temperature. The solvent was removed by gentle warmth in a stream of nitrogen. The resultant slurry was steam distilled,giving the crude yellow selenoeyanate in 70-80 ~o yield. Vacuum distillationwith a nitrogen leak yielded a pale yellow product [5]. Infrared spectra were obtained with a Perkin-Elmer 521 grating spectrometer, calibration being carried out on each r u n using t h e sharp a b s o r p t i o n bands of gases [6]. T h e frequencies are corrected to v a c u u m a n d are considered t o be a c c u r a t e t o * To whom enquiries should be made regarding this paper. [1] E. E. AYNSLEY,N. N. GREEN~VOODand M. J. SP~AGUE,J. Chem. Soc. 2395 (1965). [2] T. TARANTELLIand D. LEONESI, A~n. Chim. (Rome) 58, 1113 (1963). [3] C. COLLARD-CHARONand M. RE~SON, Bull. Soc. Chim. Belges 72, 315 (1963). [4] R. P. I~RSCH~r~, R. N. K~ISELEY and V. A. FASSEL,Spectrochim. Acta f$O, 809 (1964). [5] W. J. FttA~T~rN and R. L. WER~rER, Tetrahedron Lett. 34, 3003 {1965). [6] H. W. THO~PSON(Editor) Tables of Wavenumbers for the,Calibration of InJrared Spectrometers, Butterworths, London (1961). 387
388
W.J.
I~RANKLIlq, R. L. WERNER and R. A. ASHBY
~-1 cm -1 for sharp bands. Low temperature spectra were obtained using a cell similar to that of RICHTEL and KLXPPMEIRER [7]. The vapour spectrum of methyl selenocyanate was obtained with a conventional 1 m gas cell heated to 65°C electrically. Some slight decomposition occurred during repeated scans but this was not a serious problem. B y repeated fractionation a nearly colourless sample of methyl selenocyanate was obtained and the R a m a n spectrum was recorded of this sample using a Hilger E612 spectrograph with a photoelectric scanning attachment. Band intensities were measured in terms of peak height relative to the carbon tetrachloride 458 cm -1 line recorded under identical conditions with correction for the response of the detector. The frequencies are considered to be accurate to ~=2 cm -1. Depolarisation ratios were measured b y the method of EDSALL and WILSON [8] and corrected by the procedure of KoNmsTEIN and BERNSTEIN [9]. RESULTS AND DISCUSSION OF SPECTRA The absorption frequencies together with proposed assignments are given in Tables 3-5 inclusive. In general it m a y be said that the low temperature spectra exhibited sharper bands compared to the liquid state as, for example, has been found for the alkyl halides. As the molecular weight of the compound increased the bands increased in width and there were fewer examples of crystal splitting. Before discussing the spectra of each molecule separately it is proposed to make a comparison between them for the purpose of distinguishing those features arising from the common functional group. Alkyl selenocyanates give rise to strong absorption in the region 2140-2160 cm -1, indeed, in the case of the five compounds studied the peak was measured at 2152 q- 2 cm -1. The integrated intensity of the band is close to 1 × 103 mole -1 1 cm -~ in all cases and the half-intensity bandwidth approximately 8 cm -1. The similar absorption bands in the case of alkyl thiocyanates lie at 2156 4- 3 cm -1 [4]. The location of these bands and the negligible frequency shift occurring on replacement of the sulphur with a selenium atom indicates an essentially uncoupled CN stretching mode. In view of this, the two remaining stretching modes of the C ~ S e - - C N chain ought properly to be sought in the region of C--Se stretching vibrations. HIRSC~MA~N et al. [4] have successfully interpreted the spectra of alkyl thiocyanates in terms of separate ~(C~--S) and ~(S--CN) vibrations and this receives indirect support from the microwave data on methyl thiocyanate [10] which show that the CSC angle is close to 100 °, implying no great interaction between the C ~ S and S - - C N arms. There are no structural data on alkyl selenocyanates b u t the vapour spectrum of methyl selenocyanate is very similar to methyl thiocyanate and there is, in general, a marked correspondence between the liquid selenocyanates and the appropriate thiocyanate. The premise is adopted, therefore, that a separate C~--Se [7] [8] [9] [10]
H. It. RICHELand F. It. KL_~PI~ErRE~, Appl. Spectry 18, 113 (1964). J. T. EDSALLand E. B. WILso~r, J. Chem. Phys. 8, 124 (1938). J. A. KONIGST~I~and I-I. J. BERNSTEI~,Spectrochim. Acta 18, 1249 (1962). S. NAGAKAWA,T. KOJIM.~, S. TXmCKAJKIand C. C. LI~, J. Mol. Spectry 14, 210 (1964).
The vibrational spectra of alkyl selenooyanates
389
and Se--CN stretching mode can be identified rather than asymmetric and symmetric modes involving all four atoms. The absorption of each of the compounds in the region 4000--250 cm -1 is shown in Figs. 1-4 for both liquid and solid states. I t would be anticipated t h a t v(C~-Se) should exhibit some dependence on the number of methyl groups attached to the C= atom as is the case with thiocyanates [4], thiols [11] and alkyl halides [12] and the frequencies 578, 545 and 530 cm -z for CHsSeCN, CzHsScCN and i-CsH~SeCN accord with this. I n Table 1 these values are compared with the corresponding C~--X frequencies found for alkyl chlorides, bromides, thiocyanates and dialkyl diselenides [13]. The last named is relevant since the long Se--Se bond in conjunction with a dihedral angle believed to be close to 90 ° has the effect of isolating the C--Se bonds, giving two mutually independent v(C---Se) vibrations of coincident frequency. As can be seen there is close agreement between C=--Br and C~---Se (-Se-) frequencies in the case of selenocyanates. Unfortunately, there appears to be no recorded value for di-isopropyl disulphide to permit a further comparison of disulphide with thiocyanate frequencies. I n further support of the assignments made, bands are found at 546 and 545 cm -z (the latter as a shoulder) in the spectra of n-propyl and n-butyl selenocyanate, these values agreeing closely with the value found for ethyl selenocyanate (secondary C= in each case). The possibility of hindered internal rotation about the C~---Cp bond should not be neglected. I n the case of n-propyl and n-butyl selenocyanate this will result in rotational isomers involving the position of the alkyl group with respect to the ~ . 0 5 0 mm 0'05,
3000
2000
Liquid
I000
cm "l
--V
Solid
Fig. 1. The infrared spectra of CHsSeCN (notations on spectra refer to sample thickness). [11] 1~. SHEPPARD, Trans. Faraday Soc. 46, 429 (1950). [12] J. J. SHaMAn, V. A. FeLT and S. KRIMm, Spectrochim. Acta 18, 1603 (1962). [13] G. BERGSON, Arkiv Kemi 13, 11 (1959).
390
W. J. FP.AN~IN, R. L. W~.~NF~ and R. A. ASHBY o'oqTmm
0"0'
o.opr
)7
0"0~
o-058mm i
2000
5000
Liquid
T I000
!
c m -~
Solid
Fig. 2. The infrared speetra of iso-CaHTSeCN (notations on spectra refer to sample thickness) \
~l
%oozmm
L..9~o7~ .----I--._
5000
2000
o.oorJ
Liquid
/
[000
(;m -I
i
Solid
Fig. 3. The infrared specbra of n-C3HTSeCN (notations on spectra refer to sample thickness).
C,--Se bond. I f the barriers to internal rotation are of the usual magnitude for C--C bonds, the isomerism ought to be detectable by the observation at ordinary temperatures of discrete absorption due to ~(C~---Se)for each isomer. As pointed out by HIRSCH~KANNet aZ. [4], two C,--C~ isomers are expected provided only staggered configurations are involved. The bands at 629 and 828 cm-1 in n-propyl and nbutyl selenocyanate are assigned to ~(C,--Se) in isomeric forms. The band in
The vibrational spectra of alkyl selenoeyanates
O.O~,Tmm
391
,D'O0? -
.
3000 -007turn
0-0072000 Ethyl
~ ~,.---v
o-o3s
I000
/0.184
cm-' ~"I O.IS4
N-but"yl
Fig. 4. The infrared spectra of CsHsSeCI~ and ~-C4HgSeCN (notations on spectra refer to sample thickness). Table 1. Correlation of C=--X frequencies (all frequencies given in em-1) Compound CHsX CHsCI-I~X (CHs)sCHX
X~-----C1 X ~ B r 712 656 615
594 560 536
X--S(SCN) 675 624 584
X~Se 570 536 517
(-Ses-)
X--Se
(SeCN)
578 545 530
n-propyl selenocyanate disappears completely on cooling to --180°0 and its assignment to a less stable isomer is convincing. The band in the spectrum of the n-butyl compound still tends to persist at low temperature, b u t there were indications that a glass rather than a crystalline film had formed and this m a y account for the inconsistent behaviour. The behaviour of the corresponding ~-butyl thiocyanate is not known. The second C~-Se stretching mode, i.e. v(Se--CN), is also subject to the effect of rotational isomerism. This bond, in all the compounds other than the methyl derivative, can "see" two isomeric configurations of the remainder of the molecule b y virtue of the hindered rotation about the C=--Se bond. A system of notation for such isomers is given b y H r R S C ~ N et al. and will be used here. Inspection of Figs. 1-4 shows t h a t after making the preceding assignments, the straight chain compounds (including methyl selenocyanate) each give rise to a band near 520 cm -1 which persists at --18000. The ethyl and higher ~-alkyl selenocyanates give, in addition, bands at about 565 cm -1 which appear as pronounced shoulders at room temperature and which have no counterparts in the spectra at low temperatures. The band at 520 cm -1 is assigned, therefore, in each case to v(Se--CN) in the more stable isomeric form, while the shoulder at 565 c m - x i s assigned to the less stable
392
W.J.
FRA-~KLI~, R. L. W~.RNER and R. A. Asma~-
form. I n iso-propyl selenocyanate ~(Se---CN) corresponding to the less stable isomer probably gives rise to the shoulder at 540 cm -1 which also disappears at low temperatures in the solid state spectrum. The corresponding band of the more stable isomer is taken to be the shoulder on the low frequency side of the band due to ~(C~--Se) at 530 cm -1. In the solid state the band appears distinctly at 512 cm -1. A distinction between thiocyanates and selenocyanates is apparent in the assignments above. In the thiocyanates ~(S--CN) for the less stable isomer occurs at frequencies consistently lower than its partner [4]. The reverse is true of ~(Se--CN) frequencies. Further, the frequencies of those bands which persist in the solid state crn-* 600 t
700 I
500 I
650 I
600 I
--CH3--
i
t
--C2H 5-
-n-C3H 7 -
-iso-C3H 7-
i Selenocyano'l-es
-n -C4H 9-
Thiocyonotes
Fig. 5. Correlation of C - - X stretching vibrations [solid lines connect frequencies associated with v(Ca--X ), and broken lines those associated with u(X---C(N)). ttorizontal bars denote bands which disappear at low temperatures (no data on n-C4HgSCN)].
are the same as that of ~(Se--CN) in the methyl compound which has an H configuration. I t may be, therefore, that this particular configuration is the more stable in the selenium compounds although, on more general arguments, the alternative C configurations would be preferred. The correlation of C--Se stretching frequencies in the various alkyl selenocyanates is illustrated in Fig. 5 which m a y be compared directly with that given for the C--S stretching frequencies of the thioeyanates [4]. The formal analogy is complete and it is probable the energy differences between the C~--Se rotational isomers is of the same magnitude as for the thiocyanates. Direct measurement of these differences by intensity measurements over a range of temperatures is scarcely possible owing to the extensive overlap of the v(Se--CN) and v(C~--Se) bands. The deformation vibrations of the CSeCN unit are readily identified by comparison of the spectra of the higher homologues with methyl selenoeyanate. Two of these, namely 6'(SeCN) (in-plane) and O"(SeCN) (out-of-plane) are assigned to the bands at 410 -~ 10 cm -1 (vw) and 363 ± 2 cm -1 (w) respectively. The choice is principally made in this order in view of the fact that the lower frequency band of the methyl selenoeyanate gives rise to a Raman depolarised line. Corresponding
The vibrational spectra of alkyl selenocyanate~
393
bands have been observed in phenyl selenocyanate [1] and there is a corresponding band at 357 cm -1 in selenocyanogen [14]. The third deformation mode 6(CSeC) can be assigned in methyl selcnocyanate to the polarised R a m a n line at 166 cm -1. The corresponding deformation vibration in dimethyl selenide falls at 236 cm -1 [15]. There is, therefore, a good correlation with the corresponding sulphur compounds since 6(CSC) in methyl thiocyanate has been assigned to a band at 190 cm -1 [16] while the deformation vibration in dimethyl sulphide falls at 285 cm -1 [15]. The higher alkyl selenocyanates were not examined below 250 cm -1 but it is anticipated that similar bands near 170 cm -1 would be encountered. Comments follow on particular members of the series: (a) Methyl selenocyanate The assignments proposed (Table 3) agree essentially with those given by AY•SLEY et al. [1] whose data are, however, less extensive. The comparison of the vibrational spectra of methyl thiocyanate and methyl selenocyanate convinces one, if this were needed, of the non-linear geometry of the heavy atom skeleton. The highest symmetry available then is point group Cs; the fifteen normal modes consisting of ten of species a' (v~s'CH3, vsCH a, ~CN, O~'CHa, 8sCHs, r'CH3, vSe--C, ~Se--CN, ~'SeCN and ~CSeC) and five of species a" (~as"CHs, (~as"CHs, r"CH s, 8"SeCN and vCHs). I t might be expected however, that, as in the case of methyl thiocyanate, the vibrations ~,'(CHs) and ~"(CHs) and the corresponding deformation pair would be coincident, at least in the condensed state spectra. On this basis the bands at 3040, 2946, 1420 and 1279 cm -1 are assigned to ~a~(CHs) (a' and a"), vs(CHs), ~as(CHs) (a' and a") and 0s(CHs), in t h a t order. The position is similar to that found for methyl thiocyanate [4, 17]. The two methyl rocking modes are almost coincident, giving rise, in the liquid spectrum to a strong absorption at 929 cm -1 and a barely defined shoulder at 920 cm -1. I n the solid state spectrum at low temperatures the strong band was found at 935 cm -1 while the weaker band, now quite distinct, occurred at 925 cm -1. Both bands exhibited crystal splitting. The methyl torsional mode probably lies near 100 cm -1 by analogy with the corresponding assignment for methyl thiocyanate [18]. The band at 166 cm -1, assigned to 8(CSeC), clearly lies at too high a frequency and is, in any case, polarised in the R a m a n effect. The infrared spectra of methyl thiocyanate and methyl selenocyanate in the vapour state are similar in m a n y respects. No detailed illustrations of the vapour spectrum of methyl thiocyanate have appeared in the literature although it has been described [4]. Some band contours for the thiocyanate, obtained during the course of this work are reproduced in Fig. 6 together with contours for methyl selenocyanate. Similarities in bands which correspond in the spectra of the two compounds suggests t h a t the thiocyanate and selenocyanate have similar structures. On the [14] E. E. AYNSLEY,1~. N. GREEN'WOODand M. J. SPRA(~UE,J. Chem. Soc. 704 (1964). [15] S. SIEBEI~T,Z. AlrbO~'g.AUgem. Chem. ~71, 65 (1951). [16] F. A. MTT.T.V.Rand W. B. WHITE, Z. Eldctrochem. 84, 701 (1960). [17] R. VOGEL-HOGLER,Aeta Phys. Austriaca 1, 311 (1948). [18] W. G. FATELEYand F. A. MZLLER,Speetrochim. Acta 17, 857 (1961).
394
W.J.
FRANKLIN, R. L. WERNER and R. A. ASHBY C m -I
3050 r
2950 T
T
2200 r
2150 1
6
.o do
Fig. 6. The infrared spectrum of CHsSeCN vapour [path length, 1 m; 80°C (inserts -CH.qSCN): spectral slit width 1 cm-1].
other hand, if one assumes that the CSeC angle is about 100 °, close to the value of 99052 ' appropriate to the CSC angle of methyl thiocyanate [10] then it is true that the b a n d contours of both compounds do not consistently reflect the molecular geometry. The vapour contours predicted are essentially those of a B-type asymmetric rotor in the case of ~(CH3) and ~ ( C H 3 ) ; an A-type band for ~(CN) ; mixed A- and C-type bands for coincident ~as(CH3) and ~(CH3) pairs of vibrational modes and A- and U- type bands respectively for the a' and a" rocking modes. The contours observed for the ~s(CHs) and Os(CH3) bands do not match these predictions although the other bands could be interpreted as conforming to the predictions made. I n actual fact, the bands due to ~s(CH3) and O~(CHs) suggest structures approaching closely tJhat expected of the true symmetric top and the interpretation of these contours for U, type molecules is clearly indicated. The contour of the band(s) in the region of the rocking vibrations in the vapour spectrum can be interpreted as a near superposition of a higher frequency A-type band and a lower frequency U-type band of which only the characteristically strong central branch is evident. Thus the higher frequency is taken to be the in-plane (a') rocking mode. Although it is not possible to draw firm conclusions as to the details of bonding in methyl selenocyanate from the vapour spectrum, there is reason to doubt that there is as much - S e + - - C - - N - back bonding character as there is -S+~C~-~N - in methyl thiocyanate [12]. Confirmation of this statement comes from a comparison of the ~(C--X) and • (CN) frequency assignments of these and related compounds (see Table 2). The ~(S--CN) frequencies fall in the same range as ~(S--CHs) frequencies whereas ~(Se--CN) falls at distinctly lower frequencies than ~(Se--CH3) and the presence of considerable S ~ C character in sulphur dicyanide is definitely established
The vibrational spectra of alkyl selenoeyanat~s
395
Table 2. Comparison of ~(C---X) and ~(CN) assignments for related compounds Compound
~(C--S)
CHsSCN
699 (S---CN)
675 ( c ~ - - s ) 741, 691 689 684, 665* 673, 667
CHa-~S--CH s CHs--(S)s--CH s S(CN)s (SCN)s~
v(CN)
Ref.
2158
4
Compound
~(C--So)
CHsSoCN
521 ( S e - - C l q )
---
-
2183, 2179 2171
v(CN)
Ref.
2152
576 (C~---Se) 19 19 14 20
CHs--Se---CH s CHs--(Se)s--CH s Se(CN) s (SeCN) s
603, 568 570 516(2)~" 521(2)
2171 2152
15 13 14 14
* C o n f i r m e d in R e f . [20]. t A s s i g n m e n t t o 608 (w) a n d 516 c m -1 p a i r in [14] s e e m s u n a c c e p t a b l e .
Table 3. Frequencies [cm-l(i.r.)] and assignments for CHsSeCN Liquid
Solid (--180°C)
3957 3862 3624 3520 3432 3039 2946 2820
3958
2672 2545
2679-2671~ 2559
2512 2329
2534-2526t 2323
3044 2953 2821
2198 2191 2152 2153 2104 2101 1850 ~ 1 8 5 0
Assignment
Liquid
3039 ~- 935* ~ 3964 2946 -~- 295* ~ 3871 3039 -~- 578 ~ 3617 2946 -~ 578 ~ 3524 2152 -~- 1278 ~ 3430 ~'as CHs(2) R a . depol? Vs C H s - - R a . pol. 1420 -~- 1420 ~ 2840 2946--r CH s (~125) 2152 -~- 521 ~ 2673 ~ 2 1 5 2 -~- 401 ~ 2553 L1278 -~- 1278 ~ 2556 2152 -~ 365 ~ 2517 2152 ~- 156 ~ 2318 1420 -~- 925* ~ 2345 1278 -~ 925* ~ 2203 V C N - - R a . pol. ? 925* -~ 925* ~ 1850 1278 -}- 578 ~ 1856
1721 1616 1501
Solid ( - - 180°C)
1420 1341
1423
1278 1146 1070 929 920 811 728 578 521 401 365 1665 100§
1278 1153 946-926 822 ~750 585-580~ 518 401 371
Assignment ? ? 925* -~- 578 ~ 1503 1420 -~- ~- C H s ( ~ 8 0 ) (~as C H s ( 2 ) - - R a - depol. 925* -~- 401 ~ 1326 1420-~ CH 3 (~80) ~ C H s - - R a . pol. 578 -~- 578 ~ 1156 925* ~- 166 ~ 1091 {r' C H s I.r" C H s 401 -~ 410 ~ 802 465 ~- 365 ~ 730. ~ S e - C H a - - R a . pol. ~ S e - C N - - R a . pol. ~ S o C N - - R a . pol. ~" S o C N - - R a . pol. ~ C S o C - - R a . pol. ~" C H s
* Average rocking frequencies. ' D e n o t e s i n - p l a n e ; " o u t o f . p l a n e in r e s p e c t o f Us s y m m e t r y . t C r y s t a l field s p l i t t i n g . :~ R a m a n f r e q u e n c y . § Postulated value.
[21]. Complementary behaviour of the ~(CN) frequencies is not clearly evident; nevertheless the integrated intensities of the ~(CN) bands of selenocyanates are only about half t h a t for thiocyanates suggesting a closer approximation to a true -C~____N configuration in the former. (b) Ethyl and i-propyl selenocyanate Previous analyses of the vibrations of ethyl and iso-propyl groups [4, 22] in conjunction with the preceding discussion on the vibrations of the CSeCN group make it possible to give nearly complete assignments for these two compounds. Those proposed in Table 4 are based on an assumed Cs s y m m e t r y for the molecules; the maximum possible in either case. The low s y m m e t r y results in m a n y equivocal [19] [20] [21] [22]
D. W. SCOTTand J. P. McC~-~LOUGH,J. Am. Chem. Soc. 8 0 , 3554 (1958). M. J. NELSO~and A. I). E. PI~-~LI~,J. Chem. Soc. 604 (1960). L. PIERCE, R. NELSO~and C. THOMAS,J. Chem. Phys. 43, 3423 (1965). N. SHEPP~D, J. Chem. Phys. 17, 79 (1949) and Trans. Faraday Soc. 43, 533 (1950).
396
W.J.
FRA~r~L~, R. L. W~R~E~ and R. A. ASHBY
Table 4. Frequencies [em-~(i.r.)] and assignments for CsHsSeC~Tand iso--CaH:SeCN CsHsSeCN Liquid 365 406 522 645 ~565 754 961 1017 1054 1092" 1237 1382 1430 1449§ 1500
Solid ( - - 180°C) 374 411 517 522 750 960 1016 1054 1092 1234 1380 1433 1457 1509
1621" 1777 1917 2102 2153 2188 2330 2426 2460 2509 2549 2672 2738 2824 2871 2928 2974** 3000 3127 3252 3384 3523 3620
2102 2154 2188
2837 2870 2933 2970 3006
Assignment (~" SeCN (~P SeCN v Se--Ol~ v C~--So v Se---CNt C H s rock v C---C(and r" CHs) CH s twist r' C H s 545 --~ 545 ~ 1090 CH s wag ~s CHs CH s bend (~as CHs(2) 754 -}- 754 ~ 1508 545 -~ 961 ~ 1506 1054 -~- 565 ~ 1619 t 754 -~ 1017 ~ 1771 545 -~- 1237 ~ 1782 961 -~ 961 ~ 1922 1054 -~ 1054 --~ 2108 v 01~ 754 -~- 1430 ~-~ 2184 961 -~- 1382 ~ 2343 1054 -~ 1382 ~ 2436 1017 -~ 1449 = 2466 1054 ~- 1449 ---~ 2503 406 -[- 2153 ~-- 2559 522 -}- 2153 ~ 2675 ~ 1382 -~ 1449 ~ 2831 Vs C H s, vs CHs vas C H s vas CHs(2) ? ? 2871 ~- 365 ~ 3236 r I 5 3 -~- 1237 ~ 3390 2871 -}- 522 ~ 3393 1382 ~- 2153 ~ 3535 2671 ~- 754 ~ 3625
Liquid
A" A' A' A" A" A' A" A' .4' A' A' A'A"
A'
363 419 444 512" 530 ,~540 628~ 727 770 823 877 933 950 1042 1117 1157 1219]] 12311[ 1282 1217 1373 1389 1458 1488" 1685
iso--CaH~SeCN Solid ( - - 180°C) Assignment 369 399 445 512 535
737 765 835 884 929 947 1041 1120 1161 1229 1276 1305 1367 1389 1440¶ 1488
1750
A'A' A" A'A"
1835 1910 2100 2152 2247
2099 2150
2302 2387
2420 2506 2538 2608 2676 2732 2767 2810 2869 2897 2928 2983 3200 3230 3371
2866 2888 2924 2969~
(~" SeCN (~' SeCN ~ CCC? v Se---CI~ v Ca---So ~ So---CN~ ? 363 -~- 363 ~ 726 363 -~- 419 ~ 782
A" A' A" A' .4'
v ' C-C .4' r" C H s A" r" CH s A" r' CH s A' v" C - - C A" r ' CH s A' ~ GH A' ($ C H A" 933 -~ 363 ~ 1296 877 ~- 444 ~ 1321 950 -t- 363 ~ 1313 (~s CHs A" (~s CHs A' ~as CHs(4) A ' ( 2 ) , A"(2) (~as CHs -~- lattice v i b r a t i o n 1317 ~- 363 ~-- 1680 1157 -~- 530 ~- 1687 877 -~ 877 ---- 1754 1389 -}- 363 ----- 1752 877 ~- 9 5 0 - ~ 1827 1373 -{- 530 = 1903 877 -I- 1231 ~ 2108 950 -~- 1157 ~ 2107 v CN A' 1317 -{- 933 ~ 2260 877 -{- 1373 ~ 2250 1157 -~- 1157 ~ 2314 1231 -[- 1157 ~ 2388 1440 -}- 950 ~ 2390 1042 -}- 1373 ~ 2415 2153 ~ 363 ~ 2516 1117 ~ 1389 ~ 2506 1157 -[- 1373 ~ 2530 1377 -.[- 1231 ~ 2608 2152 --~ 530 ~ 2682 1231 -~- 1440 ~ 2671 1373 -{- 1373 ~ 2746 1373 ~ 1389 ~ 2762 1440 -~- 1370 --~ 2813 vs CHs(2) A'A" 1440 -~- 1440 = 2880 1440 -~ 1458 ~ 2898 Vas CHs(2) A"A" ~as CHs(2) A' 2152 -{- 1042 ~ 3194 363 -~ 2869 ~ 3232 2152 ~- 1231 ~ 3383
* Solid s t a t e f r e q u e n c y only. t Less stable H g (~) isomer. § B a n d s a t (1457, 1455) a n d 1443 in solid. ** 4 b a n d s in solid. il Single b a n d in solid. ¶ F u r t h e r split in solid. ~ t B a n d s a t 2986, 2963, 2945 in solid. V e r y w e a k indeed, n o t to be confused w i t h Ca~-Se b a n d a t 628 e m -1 in t h e s t r a i g h t chain c o m p o u n d s - could in fact signify t h e presence of t h e n - p r o p y l isomer as a n i m p u r i t y .
The vibrational spectra of alkyl selenocyanates
397
combination and overtone assignments. The modes not assigned are those described as ~(CCSe) (2 in either molecule), ~(CSeC), ,(CHs) (one in the ethyl and three in the iso-propyl compound) and ~(C--H) in /8o-propyl selenocyanate. Apart from • (C--H) which is expected to be weak and obscured by ~(CHs) and ~(CH~) the modes not assigned lie below the frequency range investigated and it is noted t h a t one of the 5(CCSe) modes (the out-of-plane one) must be equivalent to a turning movement about the C,---Se bond, i.e. to the motion by which isomerisation occurs. There is some inconsistency in the CH~ bending assignment for ethyl selenocyanate. Applying the correlation between CH z deformation frequencies of XCH~Y molecules with the electronegativity product, ~(X)~(Y), reported by JoN~s and THOMAS [23], to ethyl thiocyanate and ethyl selenocyanate, the CH~ twisting, wagging and rocking frequencies of the former [4] are about 1.03 times those as assigned for the latter but the ~(CHz) frequencies are virtually the same. I f ~(CH~) in ethyl selenocyanate is taken to be coincident with ~(CHs) at 1382 cm -1, a ratio of 1.032 is obtained, but concomitant assignment of the 1430 cm -1 band to one of two possible binary combinations is less t h a n satisfactory because of its considerable intensity. I n the assignments for iso-propyl selenocyanate, the a' r(CHs) and ~(C--C) assignments are revised compared to those given by SHEPPARD following the precedent set in [4]. The two ~(CH) modes are assigned to the strong doublet at 1231, 1219 cm-L There seems no alternative, since to assign one of the 5(CH) vibrations to the very weak band at 1317 cm -1 leaves the doubling of the strong band unexplained. HIRSCHWA~I~ et al. [4] have assigned the ~(CH) modes in iso-propyl thiocyanate to 1318 (w) and 1247 (s) cm -1, and exception must be taken to this in view of the results on iso-propyl selenocyanate. The illustration of their spectrum in [24] shows t h a t the 1318 cm -1 band is very weak indeed, while the 1247 cm -1 band has a shoulder at 1258 cm -1, with two strong bands at 1247 and 1258 cm -1 appearing in the solid spectrum. I t is thus tempting to reassign the 1318 cm -1 band to a combination.
(C) n-Prowl and n-butyl selenocyanate8 Fundamental assignments (Table 5) have also been attempted for n-propyl and n-butyl selenocyanate based as far as possible on the work of SHEPPAI~D and SIMPSON on n-alkanes [25]. Those in the 1350-700 cm -1 region rest on schematic descriptions of the normal modes which, at best, can be only approximately correct, and combinations and overtones of the low frequency vibrations m a y also complicate the issue by virtue of Fermi resonance interaction. I n the n-butyl compound there is the added complication of rotational isomerism about the C~--Cp bond. The low temperature solid spectrum showed no marked differences from the liquid spectrum, which, as noted before, could be attributed to the formation of a glass. Those bands (above 720 cm -1) in n-propyl and n-butyl selenocyanate which have [23] R. G. JO~rESand W. J. ORVILLE-THOMAS,~_~ctrochi~. Acta 20, 291 (1964). [24] R. P. H I ~ S C H ~ , Ph.D. Thesis, Iowa State University of Science and Technology, 1963, Diss. Abs. 64-3875. [25] 1~. SBEEPPA.RDand D. M. SIMPSON,Quart. Rev. (London) 7, 19 (1953).
398
W.J.
F R A n K L I n , R . L . WERNER a n d R . A. ASHBY
T a b l e 5. F r e q u e n e i e s [ c m - Z ( i . r . ) ] a n d a s s i g n m e n t s f o r n - C a H ? S e C N a n d n - C a H 9 S e C N n-CaH~SeCI~ Solid (-- 1 8 0 ° C )
Assignment
Liquid
363
372
~ SoCN
413
413
~' SeCN (and ~ CCCD Se--CN v C~--Se So---CN* Ca---Se*
362 38O 400 468 520 545 ~565 628 725~ 738~ 776| 786] 868, 907
Liquid
521 546 ~565 629 727 771 825?
514 548
778-771 828
r CH 2 modes
884 896
886
Ys C--C--C Ys C--C--C*
1032 1039 1076 1093
1027 1041 1078 1090
tw -CH2--Se r CH 8 r CH a ~as C--C---C
1217
1224
wag -CHf--Se
1282
1281-1278
tw -CH~-
1342
1340
wag -CH z-
1383 1420 1443 1461 2152 2959 2876 2937 2968
1380 1422 1449 1469-1454 2155 2860 2872 2940-2937 2976-2967
(~s CHa (~ -CH~--Se (~ -CH~~as CHa (2) ~ CN ~s CH~ (2) vs CH a ~asCH~ (2) Vas CH8 (2)
n-C~HsSeCN Solid (--180°C) 368 372
~ SeCN ~ CCG (~ SoCN ~ CCC 3' So---CN ~ C~--Se~ ~ Se---CN*
514 547 629 724~ 737~ 771| 748.' 865, 893, 903
v C~--Se~ r CH 2 modes v C--C--C--C*
968, 995 965, 994 1009~ 1013 10491 1047 1062 1059 1099 1098 11655 ~ 1165, 1149~
12095 l !
1261,~ 1290, 1297~}
|
1344,1351 1359, 1369 1382 1421 1439 1467 2152 2867 2877 2937 2963
| J
Assignment
v C--C--C--Ct r CH a and tw -CH~--So r CH a v C--C--C--C
1204 l 1295 1259 1304 1357 1360 1378 1419 1435 1465 2150 2871
|~ | |
tw and wag CHI modes
}
2932 2958
~s CH s (~ -CH~--So (~ -CH~(~as CH~ (2) v CN Vs CH~ vs CH 8 ~as CH~ ~as CHa (2)
* Less stable rotational isomer. ~f In different rotational isomers. 5 Probably more closely associated with -CH2--Se, by comparison with n~C4HgSCI~T. • Denotes out.of-plane; ~ in-plane with respect to CSeCN group.
appreciably different frequencies when compared to the otherwise very similar thiocyanate spectra [4] are taken to be associated mainly with the CH 2 group adjacent to the selenium atom. The O(CH2) mode again appears not to shift on exchanging the SCN group for the SeCN group.
Acknowledgements--One o f
u s (W. J . F . ) is i n d e b t e d t o t h e C . S . I . R . O . a n d t o t h e U n i v e r s i t y o f New South Wales who, in turn, granted him post-g ra dua te s tude nts hips without whic h he would have been unable to participate in the work re porte d herein.