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The electronic spectra of some dialkyl sulphides
Spectrochimica Acta, Vol. 24A, pp. 265 to 270. Pergamon Press 1968. Printed in Northern Ireland
The electronic spectra of some dialkyl sulphides J. BARRETT and M. J. HITCH Department of Chemistry, Chelsea College of Science and Technology, Manresa Road, London, S.W.3 (Received 27 M a y 1967)
Able, tot--The electronic spectra of a series of dialkyl sulphides have been recorded in n-hexane and propaff-2-ol solution. The absorption maxima, which are situated in the region 214-194 mp cannot be assigned to a single transition, the absorption being duo to an intravalence shell transition iN -~ V) and several int~rvalenee shell or Rydberg transitions iN -~ R). INTRODUCTION THERE have been several reports of the electronic spectra of simple dialkyl sulphides. FEH~ET, and CARM~CK [1] observed absorption maxima in the region of 210 mp with shoulders at 230 mp, for solutions of dialkyl sulphides in ethanol. CUMP~.R et al. [2] using the same solvent observed absorption maxima at ,--202 rap, but no shoulders are apparent in their spectra. CLARKand SnwPSON [3] have made a vacuum ultraviolet study of the vapours of various dialkyl sulphides and report complex absorption in the region 210-185 mp. The transition(s) responsible for the low wavelength absorption of simple dia]kyl sulphides have been variously assigned to Rydberg (N--~ R) transitions and/or (N--~ V) transitions. MULLIK~N [4] considered the absorption to be due to a n -~ 4s Rydberg transition, while CUMPER et a~. [ 2 ] and LEANDRI et al. [5] favour a n --~ 3d Rydberg transition. CT,ARKand Sn~rsoN [3] have shown t h a t the absorption in the low wavelength region is composed of a mixture of several Rydberg transitions and (N -~ V) transitions. The previous experimental work with solutions appears to suffer from the effects of stray radiation in the spectrophotometer used. These effects [6] alter the shapes of bands and the positions of absorption maxima. In view of the spectral inaccuracies and the uncertainties in assignments, it was decided to reinvestigate the electronic spectra of some simple dialkyl sulphides in solution. EXP~Rn~ENTAL Materials
Dimethyl, diethyl and di-n-butyl sulphides, and dimethyl sulphoxide were purchased from British Drug Houses, Poole, Dorset; di-n-propyl and di-isobutyl sulphides from E a s t m a n Kodak, Rochester, New York; d i - s e c - b u t y l and di-t-butyl [1] [2] [3] [4] [5] [6]
E. A. FEH~L and M. CAR~ACK,J. A m . Chem. Soy. 71, 84 (1949). C. W. 1~. CUraTe.R,J. F. READ and A. I. VOOEL,J . Chem. Soe. A 239 (1966). L. B. CLA~Kand W. T. SI~PSO~, J. Chem. Phys. 43, 3666 (1965). R . S. M U L L I ~ , J . Chem. Phys. 3, 506 (1935). G . LEAI~DRI, A. M.A~GIN'Iand R. PASSERII~I,J . Chem. Soe. 1386 (1957). J. BARRETTand A. L. MA~S~.LT.,Nature 187, 138 (1960). 265
266
J. BARRETT and M. J. ~_ITCH
sulphides from the Aldrich Chemical Co., Milwaukee, Wisconsin, and dimethyl sulphone from Koch Light Laboratories, Colnbrook, Bucks. All the sulphides were shown to be pure, excepting the di-n-propyl sulphide which showed a 3% impurity, by gas liquid chromatography, using a Perkin Elmer F l l Flame Ionisation Gas Chromatograph, with a DE101 column, Apiezon L being the stationary phase on a Chromasorb P support with nitrogen as the carrier gas. The solvents used were British Drug Houses' "special for spectroscopy", n-hexane and propan-2-ol.
Spectra All the spectra were recorded using a Hilger and Watts Uvispek flushed with nitrogen [6] to eliminate spurious absorption due to oxygen. The cells used were of Suprasil with an optical path length of 0.1 cm. The stray radiation was estimated by recording the transmission of 1M sodium hydroxide solution which in the region below 200 m~u, where the effect becomes appreciable, absorbs all the monochromatic radiation emerging by the normal optical path but allows transmission of stray radiation. Where necessary the spectra of the solution and pure solvent were recorded separately. RESULTS
The spectra of dimethyl, diethyl and di-t-butyl sulphides in n-hexane solution are shown in Figs. 1, 2 and 3 respectively. They are complex spectra and do not 1400 1300 1200 7 I100
IO00 9OO .u
8OO 7O0
.8 o.
6O0 5O0
o =E
400 300 200 I00 0
190
200
210
220
WovelencJth,
230
240
250
260
mp.
Fig. 1. The electronic spectrum of dimothyl sulphide in hexane solution.
The electronic spectra of some dialkyl sulphides
267
3000
_
If ~
I
,+' T E 7 2000
w
--
g
I
I I t I I
.8 1000
' uE
ti' X
o
5000
/x
-
4000
-
5000
g
I -- 2000 o.
--
I
190
J
200
2]0
220 Wavelength,
230
240
--1----1 250
260
I000
o
0 27O
m~
Fig. 2. Comparison of the vapour phase (solid line, right-hand ordinate) and hexane solution (dotted line, left-hand ordinate) spectra of diethyl sulphide. consist of single bands. It is not possible to correlate the wavelength of maximum absorption with the nature of the alkyl group. The absorption maxima and shoulders for the three above-mentioned sulphides and those for the other compounds studied are summarised in Table 1. It is to be expected that the spectra obtained for solutions of the sulphides would be broader and shifted to shorter wavelengths in comparison to the spectra of the vapours. There appears to be a good correlation between the wavelengths quoted in Table 1 for diethyl sulphide and di-t-butyl sulphide with values for absorption given by CLARK and SIMPSO~ [3] for the respective vapours. The vapour phase spectra of diethyl sulphide and di-t-butyl sulphide are replotted from Clark and Simpson's data, and are shown in Figs. 2 and 3. The comparison of the solution and vapour spectra of dimethyl sulphide is ]ess obvious since the spectrum of the vapour is highly resolved. Dimethy] sulphoxide shows an absorption maxima in aqueous solution at 208 m/~ while dimethyl sulphone in aqueous solution is transparent in the region studied. The spectra have also been recorded in propan-2-ol solution but due to high solvent absorption it was not possible to make accurate determinations below 196 m/~. With comparable absorptions there is a slight shift to shorter wavelengths in most cases.
J. BARRETT and M. J. HrrcH
268 5000
/i
i 7
4000
-
I
I I
l
I /I
i 3000-
I
i
/
I
T g 2000 i
-- 4O00
I
o
-- 3000
o o
--
iO00
2000
u
8 -- ~000
0
190
200
210
220
I
~ 230
Wavelength,
240
I
250
I 0 260
rn F
Fig. 3. Comparison of the vapour phase (solid line, right-hand ordinate) and hexane solution (dotted line, left-hand ordinate) spectra of di-t-butyl sulphide. Table 1. Wavelengths (mp) of maximum absorptions and shoulders for some dialkyl sulphides, R2S Solutions in: R Me
Et n° Pr.
n. Bu. iso. Bu. sec. Bu. t. Bu.
n-hexane 186, 185, 188, 188, 188, 185, 185,
(191), (191), 191, 191,
(195), 196, (195), 194,
(190),
(199), 198, (199), 199,
197,
propan-2-ol 201, (201), 201, (204), 200, 202,
(214) 213 (208), (210), (209), 210, (209),
(214) 214,
200, 197, 199, 199, 197 198, (202),
(210) (208)
(201) (208), 213
( ) indicates shoulder. Italic numbers indicate most intense maximum. DISCUSSION" I t would seem f r o m o u r spectral observations t o g e t h e r with those of CL~RK a n d SIM~SO~ [3] t h a t t h e a b s o r p t i o n between 210 a n d 185 m p b y dialkyl sulphides is a m i x t u r e of contributions from R y d b e r g a n d n o n - R y d b e r g transitions. T h e spectra of the sulphides studied are with little v a r i a t i o n v e r y similar e x c e p t in t h e case o f d i - t - b u t y l sulphide where t h e r e is a strong a b s o r p t i o n b a n d with 2max at 214 rap. This seems to be the only difference, t h e rest o f t h e a b s o r p t i o n being similar to t h a t of t h e o t h e r compounds.
The electronic spectra of some dialkyl sulphides
269
ARON~.~: et al. [7] from their study of dipole moments and Kerr constants for dialkyl sulphides suggested non-planar conformations in which the corresponding carbon atoms in the two alkyl groups tend to be as far apart as possible. Steric interaction would be avoided if this were so, except in the case of di-t-butyl sulphide
where an increase in the CSC angle might be expected. Dipole moment measurements [8] support this suggestion. The orbitals of dialkyl sulphides m a y be described in terms of the Walsh "AH~" diagram [9] as shown in Fig. 4, since we can consider the bonding to involve only one orbital from each alkyl group. _ i b2i
b2
Oj
OI
90
180
C~C ongte,
deg
Fig. 4. Walsh diagram for an AH~ molecule. The CSC bond angle in dimethyl sulphide is reported [10] to be 98052 ' so t h a t the most relevant part of the Walsh diagram is the left-hand region. In the molecules studied there are eight electrons to be distributed in these orbitals. Their ground state configurations will be (a 1 - - a , ) 2 ( a l s - - ~r,,)2(b2 - - a~)~(bl - - ~r~) ~.
AL-JoBOURY and TVR•ER [11] have shown by molecular photoelectron spectroscopy t h a t in H2S the highest bonding electron is 2.2 eV lower in energy t h a n the [7] [8] [9] [10] [11]
M. A~OZ~TEY,R. J. W. L~. F~.VREand J. SxxBY, J . Chem. Soc. 1167 (1963). C. W. N. Ctr~ER, J. F. READ and A. I. VOOEL,J . Chem. Soc. 5323 (1965). A. D. W~SH, J . Chem. Soc. 2260 (1953). L. PIERCE and M. I-IAYASTTI,J . Chem. -Phys. 8§, 479 (1961). M. I. AL-JOBO~Y and D. W. TU~}TER,J. Chem. See. 4434 (1964).
270
J. BA~TT
a n d M. J . HITCH
non-bonding level (b1 -- ~r,). Assuming this order of energy difference to apply to dialkyl sulphides it would be expected that transitions from the highest bonding levels to the lowest vacant levels would be of the order of 50 m/z lower in wavelength than the lowest observed transition. The two lowest (N -* V) transitions will be from the non-bonding (b1 -- ~r~) orbital to the (51 -- 5g) antibonding orbital, and from the non-bonding (b1 -- ~,) orbital to the ( ~ - 5,) antibonding orbital. The former transition which is of lower energy, and to which we assign the main absorption band in the region of 200 mp, is allowed b y considerations of orbital symmetries whereas the latter is forbidden and will be expected to be of lower intensity. An increase in the CSC bond angle would cause such transitions to move to longer wavelengths (Fig. 4), which is observed in the case of di-t-butyl sulphide. Further evidence of the involvement of non-bonding electrons is that the sulphone is transparent in the region studied, while the sulphoxide shows an absorption at 208 m~, this would be predicted, if such involvement occurred, from the latter's shallow pyramidal stereochemistry [12, 13]. Also the progressive shift to shorter wavelengths of all the spectra observed in going from the gas phase to hexane solution and finally to propan-2-ol solution supports our interpretation that all the transitions in the region studied occur from the higher of the two non-bonding orbitals. There seems to be little point in discussing the effects of substituting alkyl groups for hydrogen atoms in the methyl groups of dimethyl sulphide since any effects are masked b y the residual Rydberg absorptions. Acknowledgements W e w i s h t o t h a n k D r . I~OBBIKS o f t h e D e p a r t m e n t of C h e m i s t r y , H a t f i e l d College of T e c h n o l o g y for p e r m i s s i o n t o u s e a " U v i s p e k " i n o r d e r t h a t we s h o u l d h a v e a c h e c k on our observations by using more than one instrument. One o f u s (M. J . H . ) is gra~eful ~o t h e Science R e s e a r c h Council for a r e s e a r c h sbuden~ship.
[12] P. V~..ALLENand L. E. SUTTO~,Acta Cryst. 3, 46 (1950). [13] O. BASTLA_~SE~and H. VrERVOLL,Acta Chem. Scand. 2, 702 (1948).
The electronic spectra of some dialkyl sulphides J. BARRETT and M. J. HITCH Department of Chemistry, Chelsea College of Science and Technology, Manresa Road, London, S.W.3 (Received 27 M a y 1967)
Able, tot--The electronic spectra of a series of dialkyl sulphides have been recorded in n-hexane and propaff-2-ol solution. The absorption maxima, which are situated in the region 214-194 mp cannot be assigned to a single transition, the absorption being duo to an intravalence shell transition iN -~ V) and several int~rvalenee shell or Rydberg transitions iN -~ R). INTRODUCTION THERE have been several reports of the electronic spectra of simple dialkyl sulphides. FEH~ET, and CARM~CK [1] observed absorption maxima in the region of 210 mp with shoulders at 230 mp, for solutions of dialkyl sulphides in ethanol. CUMP~.R et al. [2] using the same solvent observed absorption maxima at ,--202 rap, but no shoulders are apparent in their spectra. CLARKand SnwPSON [3] have made a vacuum ultraviolet study of the vapours of various dialkyl sulphides and report complex absorption in the region 210-185 mp. The transition(s) responsible for the low wavelength absorption of simple dia]kyl sulphides have been variously assigned to Rydberg (N--~ R) transitions and/or (N--~ V) transitions. MULLIK~N [4] considered the absorption to be due to a n -~ 4s Rydberg transition, while CUMPER et a~. [ 2 ] and LEANDRI et al. [5] favour a n --~ 3d Rydberg transition. CT,ARKand Sn~rsoN [3] have shown t h a t the absorption in the low wavelength region is composed of a mixture of several Rydberg transitions and (N -~ V) transitions. The previous experimental work with solutions appears to suffer from the effects of stray radiation in the spectrophotometer used. These effects [6] alter the shapes of bands and the positions of absorption maxima. In view of the spectral inaccuracies and the uncertainties in assignments, it was decided to reinvestigate the electronic spectra of some simple dialkyl sulphides in solution. EXP~Rn~ENTAL Materials
Dimethyl, diethyl and di-n-butyl sulphides, and dimethyl sulphoxide were purchased from British Drug Houses, Poole, Dorset; di-n-propyl and di-isobutyl sulphides from E a s t m a n Kodak, Rochester, New York; d i - s e c - b u t y l and di-t-butyl [1] [2] [3] [4] [5] [6]
E. A. FEH~L and M. CAR~ACK,J. A m . Chem. Soy. 71, 84 (1949). C. W. 1~. CUraTe.R,J. F. READ and A. I. VOOEL,J . Chem. Soe. A 239 (1966). L. B. CLA~Kand W. T. SI~PSO~, J. Chem. Phys. 43, 3666 (1965). R . S. M U L L I ~ , J . Chem. Phys. 3, 506 (1935). G . LEAI~DRI, A. M.A~GIN'Iand R. PASSERII~I,J . Chem. Soe. 1386 (1957). J. BARRETTand A. L. MA~S~.LT.,Nature 187, 138 (1960). 265
266
J. BARRETT and M. J. ~_ITCH
sulphides from the Aldrich Chemical Co., Milwaukee, Wisconsin, and dimethyl sulphone from Koch Light Laboratories, Colnbrook, Bucks. All the sulphides were shown to be pure, excepting the di-n-propyl sulphide which showed a 3% impurity, by gas liquid chromatography, using a Perkin Elmer F l l Flame Ionisation Gas Chromatograph, with a DE101 column, Apiezon L being the stationary phase on a Chromasorb P support with nitrogen as the carrier gas. The solvents used were British Drug Houses' "special for spectroscopy", n-hexane and propan-2-ol.
Spectra All the spectra were recorded using a Hilger and Watts Uvispek flushed with nitrogen [6] to eliminate spurious absorption due to oxygen. The cells used were of Suprasil with an optical path length of 0.1 cm. The stray radiation was estimated by recording the transmission of 1M sodium hydroxide solution which in the region below 200 m~u, where the effect becomes appreciable, absorbs all the monochromatic radiation emerging by the normal optical path but allows transmission of stray radiation. Where necessary the spectra of the solution and pure solvent were recorded separately. RESULTS
The spectra of dimethyl, diethyl and di-t-butyl sulphides in n-hexane solution are shown in Figs. 1, 2 and 3 respectively. They are complex spectra and do not 1400 1300 1200 7 I100
IO00 9OO .u
8OO 7O0
.8 o.
6O0 5O0
o =E
400 300 200 I00 0
190
200
210
220
WovelencJth,
230
240
250
260
mp.
Fig. 1. The electronic spectrum of dimothyl sulphide in hexane solution.
The electronic spectra of some dialkyl sulphides
267
3000
_
If ~
I
,+' T E 7 2000
w
--
g
I
I I t I I
.8 1000
' uE
ti' X
o
5000
/x
-
4000
-
5000
g
I -- 2000 o.
--
I
190
J
200
2]0
220 Wavelength,
230
240
--1----1 250
260
I000
o
0 27O
m~
Fig. 2. Comparison of the vapour phase (solid line, right-hand ordinate) and hexane solution (dotted line, left-hand ordinate) spectra of diethyl sulphide. consist of single bands. It is not possible to correlate the wavelength of maximum absorption with the nature of the alkyl group. The absorption maxima and shoulders for the three above-mentioned sulphides and those for the other compounds studied are summarised in Table 1. It is to be expected that the spectra obtained for solutions of the sulphides would be broader and shifted to shorter wavelengths in comparison to the spectra of the vapours. There appears to be a good correlation between the wavelengths quoted in Table 1 for diethyl sulphide and di-t-butyl sulphide with values for absorption given by CLARK and SIMPSO~ [3] for the respective vapours. The vapour phase spectra of diethyl sulphide and di-t-butyl sulphide are replotted from Clark and Simpson's data, and are shown in Figs. 2 and 3. The comparison of the solution and vapour spectra of dimethyl sulphide is ]ess obvious since the spectrum of the vapour is highly resolved. Dimethy] sulphoxide shows an absorption maxima in aqueous solution at 208 m/~ while dimethyl sulphone in aqueous solution is transparent in the region studied. The spectra have also been recorded in propan-2-ol solution but due to high solvent absorption it was not possible to make accurate determinations below 196 m/~. With comparable absorptions there is a slight shift to shorter wavelengths in most cases.
J. BARRETT and M. J. HrrcH
268 5000
/i
i 7
4000
-
I
I I
l
I /I
i 3000-
I
i
/
I
T g 2000 i
-- 4O00
I
o
-- 3000
o o
--
iO00
2000
u
8 -- ~000
0
190
200
210
220
I
~ 230
Wavelength,
240
I
250
I 0 260
rn F
Fig. 3. Comparison of the vapour phase (solid line, right-hand ordinate) and hexane solution (dotted line, left-hand ordinate) spectra of di-t-butyl sulphide. Table 1. Wavelengths (mp) of maximum absorptions and shoulders for some dialkyl sulphides, R2S Solutions in: R Me
Et n° Pr.
n. Bu. iso. Bu. sec. Bu. t. Bu.
n-hexane 186, 185, 188, 188, 188, 185, 185,
(191), (191), 191, 191,
(195), 196, (195), 194,
(190),
(199), 198, (199), 199,
197,
propan-2-ol 201, (201), 201, (204), 200, 202,
(214) 213 (208), (210), (209), 210, (209),
(214) 214,
200, 197, 199, 199, 197 198, (202),
(210) (208)
(201) (208), 213
( ) indicates shoulder. Italic numbers indicate most intense maximum. DISCUSSION" I t would seem f r o m o u r spectral observations t o g e t h e r with those of CL~RK a n d SIM~SO~ [3] t h a t t h e a b s o r p t i o n between 210 a n d 185 m p b y dialkyl sulphides is a m i x t u r e of contributions from R y d b e r g a n d n o n - R y d b e r g transitions. T h e spectra of the sulphides studied are with little v a r i a t i o n v e r y similar e x c e p t in t h e case o f d i - t - b u t y l sulphide where t h e r e is a strong a b s o r p t i o n b a n d with 2max at 214 rap. This seems to be the only difference, t h e rest o f t h e a b s o r p t i o n being similar to t h a t of t h e o t h e r compounds.
The electronic spectra of some dialkyl sulphides
269
ARON~.~: et al. [7] from their study of dipole moments and Kerr constants for dialkyl sulphides suggested non-planar conformations in which the corresponding carbon atoms in the two alkyl groups tend to be as far apart as possible. Steric interaction would be avoided if this were so, except in the case of di-t-butyl sulphide
where an increase in the CSC angle might be expected. Dipole moment measurements [8] support this suggestion. The orbitals of dialkyl sulphides m a y be described in terms of the Walsh "AH~" diagram [9] as shown in Fig. 4, since we can consider the bonding to involve only one orbital from each alkyl group. _ i b2i
b2
Oj
OI
90
180
C~C ongte,
deg
Fig. 4. Walsh diagram for an AH~ molecule. The CSC bond angle in dimethyl sulphide is reported [10] to be 98052 ' so t h a t the most relevant part of the Walsh diagram is the left-hand region. In the molecules studied there are eight electrons to be distributed in these orbitals. Their ground state configurations will be (a 1 - - a , ) 2 ( a l s - - ~r,,)2(b2 - - a~)~(bl - - ~r~) ~.
AL-JoBOURY and TVR•ER [11] have shown by molecular photoelectron spectroscopy t h a t in H2S the highest bonding electron is 2.2 eV lower in energy t h a n the [7] [8] [9] [10] [11]
M. A~OZ~TEY,R. J. W. L~. F~.VREand J. SxxBY, J . Chem. Soc. 1167 (1963). C. W. N. Ctr~ER, J. F. READ and A. I. VOOEL,J . Chem. Soc. 5323 (1965). A. D. W~SH, J . Chem. Soc. 2260 (1953). L. PIERCE and M. I-IAYASTTI,J . Chem. -Phys. 8§, 479 (1961). M. I. AL-JOBO~Y and D. W. TU~}TER,J. Chem. See. 4434 (1964).
270
J. BA~TT
a n d M. J . HITCH
non-bonding level (b1 -- ~r,). Assuming this order of energy difference to apply to dialkyl sulphides it would be expected that transitions from the highest bonding levels to the lowest vacant levels would be of the order of 50 m/z lower in wavelength than the lowest observed transition. The two lowest (N -* V) transitions will be from the non-bonding (b1 -- ~r~) orbital to the (51 -- 5g) antibonding orbital, and from the non-bonding (b1 -- ~,) orbital to the ( ~ - 5,) antibonding orbital. The former transition which is of lower energy, and to which we assign the main absorption band in the region of 200 mp, is allowed b y considerations of orbital symmetries whereas the latter is forbidden and will be expected to be of lower intensity. An increase in the CSC bond angle would cause such transitions to move to longer wavelengths (Fig. 4), which is observed in the case of di-t-butyl sulphide. Further evidence of the involvement of non-bonding electrons is that the sulphone is transparent in the region studied, while the sulphoxide shows an absorption at 208 m~, this would be predicted, if such involvement occurred, from the latter's shallow pyramidal stereochemistry [12, 13]. Also the progressive shift to shorter wavelengths of all the spectra observed in going from the gas phase to hexane solution and finally to propan-2-ol solution supports our interpretation that all the transitions in the region studied occur from the higher of the two non-bonding orbitals. There seems to be little point in discussing the effects of substituting alkyl groups for hydrogen atoms in the methyl groups of dimethyl sulphide since any effects are masked b y the residual Rydberg absorptions. Acknowledgements W e w i s h t o t h a n k D r . I~OBBIKS o f t h e D e p a r t m e n t of C h e m i s t r y , H a t f i e l d College of T e c h n o l o g y for p e r m i s s i o n t o u s e a " U v i s p e k " i n o r d e r t h a t we s h o u l d h a v e a c h e c k on our observations by using more than one instrument. One o f u s (M. J . H . ) is gra~eful ~o t h e Science R e s e a r c h Council for a r e s e a r c h sbuden~ship.
[12] P. V~..ALLENand L. E. SUTTO~,Acta Cryst. 3, 46 (1950). [13] O. BASTLA_~SE~and H. VrERVOLL,Acta Chem. Scand. 2, 702 (1948).