1980
J. Phys. Chem. 1982, 86, 1960-1966
Electric Dichroism Spectroscopy in the Vacuum Ultraviolet. Thietane, Tetrahydrothiophene, and Tetrahydrothiopyran Danlel D. Altenloh and 8. R. Russell" Department of Chemistry. North Texas State University, Denton, Texas 76203 (Received: August 18, 198 1; I n Final Form: January 5, 1982)
Electric field data are presented for the four-, five-, and six-member cyclic sulfides. The electric field technique allows determinations of certain transition and excited-state properties for molecules. Included among these properties are the change in the dipole moment, excited-state dipole moment, the change in polarizability, excited-state polarizability, and the polarization of the transition moment for absorptions. The particular absorptions studied were the low-energy Rydberg transitions identified as the s, p, and d Rydberg absorptions occurring in the near-vacuum-ultravioletregion. From these data obtained by this technique for these compounds and thiirane, several conclusions were made concerning the role of the sulfur atom in the bonding within the ring. Additionally, these data were useful in characterizing the differences in these three types of transitions with respect to electronic properties associated with the excited states for these molecules.
Introduction Using intense electric fields to partially orient polar gas molecules, a technique has been developed to obtain certain excited-state parameters. This technique, electrochromism, has been described in previous artic1es.l" The particular properties determined are the change in the dipole moment between the ground and excited states, corresponding changes in the polarizability, and the polarization of the transition. These parameters are used to help characterize the first s, p, and d Rydberg absorptions of the four-, five-, and six-member cyclic sulfides. These compounds are thietane, tetrahydrothiophene (THT), and tetrahydrothiopyran (pentamethylene sulfide; PMS). These cyclic sulfides have received considerable attention in the l i t e r a t ~ r e . ~ ~From & l ~ the photoelectron spectra (PES) and vacuum-ultraviolet spectra, the highest occupied orbital has been determined to be the lone-pair p(bJ orbital on the sulfur atom pendicular to the carbon-sulfur-carbon plane.l2-I6 Several theoretical calculations indicate that this orbital is >90% localized on the sulfur atom.'J'JO Recordings of the absorption spectra are reproduced in Figures 1-3. The absorptions investigated are labeled as s, p, or d according to the reported Rydberg (1)J. D. Scott and B. R. Russell, J . Chem. Phys., 63,3243 (1975). (2)G.C. Causley and B. R. Russell, J. Chem. Phys., 72,2623 (1980). (3)D. D. Altenloh and B. R. Russell, Chem. Phys. Lett. 77,217(1981). (4)W. Liptay in 'Excited States", Vol. 1, E. C. Lim, Ed., Academic Press, New York, 1974,p 129,and references therein. (5)H. Labhart in "Advances in Chemical Physics", Vol. 13,I. Prigogine, Ed., Interscience, New York, 1967,p 179,and references therein. (6)L. B. Clark and W. T. Simpson, J . Chem. Phys., 43,3666(1965). (7)G. L.Bendazzoli, G. Gottarelli, and P. Palmieri, J.Am. Chem. SOC., 96,11 (1974). (8)H. Sakai, T.Yamabe, H. Kato, S. Nagata, and K. Fukui, Bull. Chem. SOC.Jpn., 48,33 (1975). (9)J. A. B. Whiteside and P. A. Warsop, J . Mol. Spectrosc., 29, 13 (1969). (10)D.R. Williams and L. T. Kontnik, J . Chem. SOC.B, 312 (1971). (11)M. B. Robin, "Higher Excited States of polyatomic Molecules", Vols. 1 and 2,Academic Press, New York, 1974. (12)A. A. Planckaert, J. Doucet, and C. Sandorfy, J. Chem. Phys., 60, 4846 (1974). (13)P. D. Mollere and K. N. Houk, J . Am. Chem. Soc., 99,3226 (1977). (14)S. Pignataro and G. Distefano, Chem. Phys. Lett., 26,356 (1974). (15)D. C. Frost, F. G. Herring, A. Katrib, C. A. McDowell, and R. A. N. McLean, J. Phys. Chem., 76,1030 (1972). (16)S. D. Thompson, D. G. Carroll, F. Watson, M. O'Donnell, and S. P. McGlynn, J. Chem. Phys., 45,1367 (1966).
0022-3654/82/2086-1960$0 1.25/0
TABLE I : Compilation of Data for the Cyclic Sulfidesa
PMS
thietane
THT
69670
68380
67616
43500 49000
43000 48000 53500
44700 47990 (50930) 54260
26170 20670 1.85
25400 20400 14900 1.73
23000 19700 (16750) 13420 1.78
8.52
10.3
12.2
1.847 1.549 1.090
1.839 1.536 1.120
1.832 1.533 1.095
76.8 112 26
93.4 107.5 14.8
99.2 108.5 36.0
ionization potential, cm" transition energy, cm-' 4s 4P (4P') 3d term value, em-' 4s
4P (4P')
3d ground-state dipole moment, D ground-state mean polarizability, .A bond lengths,b A
c-s c-c
C-H bond angles,t deg
c-s-c
H-C-H dihedral
Data taken from ref 6, 11, and 23-26. Ethylene sulfide: ground-state dipole moment, 1.84 D ; bond length, C-H = 1.078 A ; bond angle, H-C-H = 116".
assignments.6J1J2 These assignments are supported by the electrochromism data and will be discussed in more detail below. The primary objective for the electric field studies was to determine whether the s, p, and d Rydberg transitions could be distinguished with respect to the differences in the excited-state properties. By comparisons of the excited-state parameters for the cyclic sulfide series, including previous data for thiirane,3 with the data for the cyclic ketones,'S2 it is possible to determine the characteristics for each particular transition. Experimental Section
All compounds used were obtained from Aldrich Chemical with purities of >99%. The samples were degassed and distilled under vacuum before use. The sample pressures were sufficient to give a maximum absorbance of 1-1.2 absorbance units (0.1 torr). All spectra were re@ 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 1981
Electric Field Data for Cyclic Sulfides
y
0. 2 30
,
I
I
I
21 8
206
194
WAVELENGTH
I
(nm)
Figure 1. Partial absorption spectrum of thietane.
IO.-
b
I P 1
ql3.
0 x E
W
0..
2i o
210
160
WAVELENGTH (nm)
Figure 2. Partial absorption of tetrahydrothiophene.
Figure 3. Partial absorption spectrum of pentamethylene sulfide.
corded with a 1-m normal incidence Model RS 225 McPherson monochromator using a hydrogen discharge lamp as the light source. A compilation of data for the cyclic sulfides is given in Table I. The electrochromism data were obtained with an orienting electric field of 200000 V cm-’. With this field strength, an arc-suppressor gas is necessary to prevent a field breakdown due to corona discharge. For this purpose, sulfur hexafluoride (SF,) was added (99.99% purity; research grade, Big Three Industries). In a preliminary study, the values obtained for the properties were found to be affected by the pressure of the arc-suppressor gas SF@’’ Work is underway to investigate
Figure 4. Experimental configuration for electrochromism. H, is the hydrogen discharge lamp; M is the monochromator; SM is a spherical mirror used to collimate the monochromator output; POL is a Wdlaston prism; PEM is a photoelastic modulator used to simulate unpolarized light: P is a photomuttipller tube (EM1 9635426): E is the current-tovoltage converter (electrometer):f is a high Q band-pass fitter tuned to 73 Hz; AC is the ac source (73 Hz); DC is the dc source; L is the logarithmic ratiometer to convert the intensity to absorption: D is an analog differentiator; DIV is an Analog Devices twoquadrant divider. The ac voltage is generated by the lock-in amplifier and is stepped up by a “Tiger-80’’ amplifier (Southwest Technical Products) and a transformer whose frequency is centered at 73 Hz.
this pressure dependence more thoroughly. However, to maintain consistency among the data, we recorded all spectra at a pressure of 4.4 atm of SF6. This pressure is slightly above the minimum required to stabilize the electric field. The vapor-phase spectra (Figures 1-3) and all electric field spectra (Figures 5-7) were obtained by using an apparatus which has been described previ~usly.’-~Certain modifications were made to interface to a microprocessor (Apple II+). Figure 4 shows the current design and associated electronics, with the various symbols defined in the legend. In general, the electrometer (E) provides the currentto-voltage conversion of the photomultiplier (P; EM1 9635 (QB) output. At this field strength, the signal-to-noise ratio for the eletric field signal is on the order of 1:lO. Thus, certain signal-enhancement techniques were necessary to separate and amplify the signal. By oscillating the electric field and using the phase coherence of the signal with the ac frequency, the electric field signal was amplified using a phase-sensitive detector (lock-in amplifier; PAR Model 124 A). These data were accumulated and converted to molecular parameters through computer programs.
Results Electrochromism, the change in the absorption band profile due to the field effect, gives rise to the following expression: 1,295
-lo”’ I f / P dv =
C1
j”’ A dv + C2 lo”’ A’dv + C3 lo”’ A ” dv (1) ”0
The electric field signal If/P is a ratio of the measured intensities at a wavelength with the field presence I f to the field absent Io. The I f is obtained from the lock-in amplifier output and the P is obtained directly from the (17) G . C. Causley, Ph.D. Dissertation, North Texas State University, Denton, TX, 1978.
1062
TABLE 11: Experimental Coefficients for t h e 202.7-nm Band of Thietane ( 4 p + n)aBb
C,, c m - '
C,
NPE (-2.4 UPE (-3.0
i i
2.8) x 2.0) x 10."
(-3.3 (-2.6
i
3.3) x
+ 3.8) x lo',
-9.4 I 3.9 -11.9 i 1 . 5
TABLE 111: Experimental Coefficients for Tetrahydrothiophene4b
C,, c m - '
Cl
UPE
209.4-nm Band ( 4 p + n ) (-1.8 I 1 . 7 ) X (3.1 i 5.4) X 10-4 10-3 (2.2 i 2.2) X (-3.7 i 1 . 9 ) X
C,, cm-,
C,
NPE
(1.0
UPE
(-3.8 i 1 0 -5
NF'E
(-2.4
(-6.3 i 10-4 (--4.1 2 4.4) 10-4
X
(-3.6 i 1 8 . 0 ) X 10-2
C,, cm-2
223.7-nm Band (4s +- n ) 1.0) X (-1.5 i 1.0) X 10'' 4.0) X ( - 4 . 3 + 4.0) X 10.'
208.4-nm Band ( 4 p +- n ) 2.0) X (-3.8 i 2.0) X 10-4 10-2 (-3.8 i 4.0) X (-4.3 i 4.0) X 10-4 10-3
UPE
-6.6
i i
0.6
f
196.3-nm Band ( 4 p n ) (-5 X ( 4 x 10-2) (-5 x 10-4) (7 X 1 0 - 2 ) 184.3-nm Band (3d +- n ) (-1.4 + 0.5) X (-5.6 i 2.8) X 10-4 10.' (-4.7 i 2.0) X (-2.2 i 0.5) X 10-4 lo-'
0.8
NPE
UPE NF'E
+
NPE
i
C,, cm-l
-32.2
i
4.2
-23.0
i
1.2
-16.3
i
4.1
-13.4
i
7.0
(-4.7) (-3.4)
+-
-3.5
10.~
186.9-nm Band (3d n ) 1.7) X (-5.0 i 6.0) X
TABLE IV: Experimental Coefficients for Pentamethylene Sulfideqb
C,, c m - ,
a NPE = 90" polarized light electrochromism; UPE = unpolarized light electrochromism. Uncertainties were calculated a t the 95% confidence level.
NPE
Altenloh and Russell
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
-32.1
i
1.5
-23.7
i
1.3
NPE = 90" polarized light electrochromism; UPE = unUncertainties were calpolarized light electrochromism. culated a t the 95% confidence level.
photomultiplier detector output. The actual ratio is on the order of about This signal is related to the absorption and the first and second derivatives of the adsorption formed from the logarithmic conversion of the intensity (Io). The coefficients represent terms involving the groundand excited-state properties. A detailed description of the coefficients including all constants has been given previously.'r2 The parametric forms of the coefficients are as follows: C1= f(ground-state dipole moment (pg8),polarization of the transition moment (e), first- and second-order C2 = field effect perturbation terms (R('),R(2),S(l),Sc2)); f(ground-state dipole moment (p& dipole moment change ( a ) , polarizability change ( b ) , field effect perturbation terms, polarization of the transition moment); C3= f(dipo1e moment change (a),polarization of the transition moment). The C3 coefficient is a function of the change in the dipole moment ( a ) for the two states involved in the transition. This property is the simplest to determine and has been shown to be the most reproducible.2 The measured dipole moment change is generally reproducible to *5%. The two coefficients, C1and C2, are complicated and represent terms involving several ground- and excited-state quantities. These quantities include the dipole moment and mean polarizability changes from the ground state to the excited state and certain first- and secondorder perturbation terms. The large relative uncertainties of these coefficients result in corresponding large uncertainties in the quantities. A least-squares fit of the electric field data to the absorption data is performed to generate values for these coefficients, which are then used to calculate the various quantities to be discussed. The coefficients generated by this method are listed with their errors at the 95% confidence level in Table 11-IV. By using two polarizations of light, and certain simplifying assumptions, one can determine the excited-state quantities. These quantities are listed in Table V-VII, together with the errors at the 95 % confidence level. Thietane This compound belongs to the C, point group. For comparisons with the other cyclic sulfides, ethylene sul-
WE
(-65) (-4 2 )
a NF'E = 90" polarized light electrochromism; UPE = unpolarized light electrochromism. Uncertainties were calculated a t the 95% confidence level.
TABLE V : Determined Excited-State Parameters for s Rvdbere Transitionsaib
a I-1 ex
e
R' b Oi ex
ethylene sulfide (212.2 n m ) C
PMS (223.7 n m )
2.7 i 0 . 5 -1.1 i 0.5 (90) 2.2 x (-150) (150)
4.1 f 0.2 -2.3 i 0.2 (90) -1.3 X 10 -200 ? 180 200 f 180
a a = p R s- p e x is t h e dipole moment change (D); pex is the excited-state dipole moment ( D ) ; 8 is the angle between the transition moment vector and dipole moment change (deg); R ( ' )is a perturbation term (esu cm erg-'); b = a g s a e x is the mean polarizability change ( A ' ) ; oex is Uncertainties when the excited-state polarizability ( A'). determined indicate the standard error of the mean at the 95% confidence level. Data for ethylene sulfide o b tained from ref 3. -
fide: and higher members, it is assumed that the C2"point group can be used to emphasize the local symmetry for the C-S-C unit. This local symmetry is suggested by the similarity of the spectrum of thietane (Figure 1) to the spectra of the other sulfides with respect to the Rydberg absorptions. These similarities for the sulfides have also been noted by Clark.6 The 226-nm absorption (4s n; s Rydberg transition) is composed of numerous vibronic components. The origin for this system has been identified at 226.5 nm.9 The structure for this band has been interpreted as an excitation to a planar state. Th geometry of the molecule is changed from having a dihedral angle of about 30° to a planar geometry. The electric field spectra of the origin and several vibrational components were measured. The same coefficients and, hence, the same dipole moment changes were obtained for all vibronic members investigated; thus, changes in the excited-state properties due to vibrational effects were less than the experimental error in the technique. The change in the dipole moment was determined to be 0.8 D and is listed in Table V together with the data for the other s Rydberg transitions investigated for the other cyclic sulfide compounds. One of the assumptions in the analysis of the electrochromism data is that there is no geometry change upon excitation. A geometry change from a "bent" to a "planar"
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The Journal of Physical Chemistry, Vol. 86, No. 1 1, 1982
Electric Field Data for Cyclic Sulfides
1963
TABLE VI: Determined Excited-State Parameters for p Rydberg TransitionsaBb PMS
ethylene sulfide (192.2 n m ) c
a I.( ex
e
R
b
ex
thietane (202.7 n m )
2.9 f 0 . 3 -1.3 i 0.5
2.8 i 0.5 -1.1 * 0.5 (0) (5.5 i 4.5) x 10 -150 i 300 150
(01 2.2 x 10 (- 400) 400
THT (209.4 n m )
208.4 nm
1.9 t 0.2 -0.2 i 0.2
196.3 nm
(1.6) (0.2)
(5.7 (-3.9)
(-130) 130
(-400) 400
(0)
(6.6 * 3.6) x 10 -70 i 1 5 0 70
a = pss
- pex is the dipole moment change (D); p e x is the excited-state dipole moment (D); 8 is t h e angle between the transition moment vector and dipole moment change (deg); R(')is a perturbation term (es cm erg-'); b = o l g s - 01 e? is the Uncertainties when determined indicate the mean polarizability change ( A 3 ) ; aex is the excited-state polarizability (A3). Data for ethylene sulfide obtained from ref 3. standard error of the mean at the 95% confidence level. (I
TABLE VII: Determined Excited-State Parameters for d Rydberg Transitions4
-6
ethylene sulfide (174.0 n m ) THT (186.9 n m ) 5.6 t 0.3 -3.1 * 0.3 (83 k 1 7 ) 4.3 x 1 0 -1560 ? 360 1560
a pex
8
R
b o!ex
4.2 i 0.1 -2.5 0.1 (90) (9.6 t 7.0) x 10 -420 650 4 20
*
*
';b r;
PMS (184.3 n m )
-4
3.1 f. 0.5 -1.3 * 0.5
-2
(9.4 i 4.3) x 1 0 (-400) 400
a = p g s - pex is t h e dipole moment change (D); p e x is the excited-state dipole moment (D); e is the angle between t h e transition moment vector and dipole moment change (deg); R(')is a perturbation term (esu cm erg-'); b = oigs - aex is the mean polarizabilit change ( A 3 ) ; aeX is t h e excited-state polarizability ( A3). Uncertainties when determined indicate the standard error of the mean at the Data for ethylene sulfide ob9 5 % confidence level. tained from ref 3.
E
1.5-
0
-0
0-
1
configuration in a Rydberg state would be expected to alter the analysis of this data. Since this transition involves a known geometry change upon excitation, comparison of this data to that for the other compounds must be done with the awareness of the possible effects of the geometry change. The 202.7-nm band of thietane has been assigned as the 4p n (p) Rydberg transition.6J1 This excited state has also been shown to be n ~ n p l a n a r . An ~ example of the unpolarized light electrochromismspectrum ( - I f / P UPE) and the 90" polarized light spectrum (-If/l");NPE) with respect to the electric field are given in Figure 5 for the 202.7-nm band. The difference in the intensity between the spectra supports an assignment in which the transition moment is polarized parallel to the major dipole axis. This data support a 4p(bl) n(bl) (A, A,) Rydberg assignment for the absorption. Using the values for the coefficients given in Table 11, we obtained a zero angle between the polarization of the transition and Cz axis. The change in the dipole moment and the mean polarizability change between the states are listed as a and b, respectively. The determination of both the magnitude and sign of these properties from the coefficients has been described.' These quantities are defined by the relations a = p g s - pex and b = cygs - cyer where the subscripts refer to the ground state and the excited state, respectively. Since the excited state is expected to have a larger mean polarizability than the ground state, the value for aexis expected to be larger than a@, resulting in a negative value for b. With this assumption, the magnitude and the sign associated with b were used to calculate the sign for a. This method has been used in calculating the excited-state data for certain ketones2 and sulfide^.^ From the value of C3, the change in the dipole moment for the 4p n Rydberg transition is determined to be 12.8 f 0.51 D. The change in the polarizability ( b ) is determined +
-
-
+
1
203
I
2 I2
WAVELENGTH (nm) Flgure 5. Examples of the absorption, unpolarized light electrochromism ( - I f / I o ) ,and 90" polarized light electrochromism ( - I f / I o ) spectra for the 202.7-nm band of thetane. The molar absorptivity (e,,) for the absorption spectrum is in 1 mol cm-'.
to be -150 f 300 A3, giving a value for the excited-state polarizability of about 150 A3. From the value and the sign for the polarizability change, the excited-state dipole moment is determined to be -1.1 f 0.5 D. The negative value is interpreted as electron density being shifted from the sulfur end of the molecule to the carbon end, resulting in localization of the positive charge toward the sulfur atom. This change in the dipole moment is the same direction and similar in magnitude to that determined for the ketone dataa2 This approach was used in the determination of the excited-state data for the subsequent absorptions studied.
Tetrahydrothiophene Tetrahydrothiophene belongs to the C, point group. From the similarity of the spectrum given in Figure 2 to the other cyclic sulfides, this compound will be discussed in terms of the CZupoint group. Microwave data support the local symmetry aspect in that the molecule has a C2 axis with the sulfur atom on the major axis.18 The 4s n Rydberg absorption is broad and unstructured, and the electric field signal was not feasible for study. The two absorptions labeled in Figure 2 assigned to be the 4p n (p) and the 3d n (d) transitions were
-
+
-
(la) A. Kh. Mamleev and N. M. Pozdeev, Zh. Strukt. Khim., 10,747 (1969).
1964
The Journal of Physical Chemistry, Vol. 86, No. 11, 7982
Altenloh and Russell
2.
.
.
. . ..
0.
1..
0. -1..
-2.
2.
1. 0. -1..
.
,
187.6
186.6
WAVEGGTH (nm)
Figure 7. Examples of the absorption, unpolarized light electrochromism ( - I c / I o ) , and 90" polarized light electrochromism (-I,/Io) spectra for the 186.9-nm band of tetrahydrothiophene. The molar absorptivity (em) for the absorption spectrum is in 1 mol-' cm-'. I , 210
I
269
2b8
WAVELENGTH (nm) Figure 6. Examples of the absorption, unpolarized light electrochromism (-I,/Io), and 90" polarized light electrochromism ( - I , / I o ) spectra for the 209.4-nm band of tetrahydrothiophene. The molar absorptivity (em) for the absorption spectrum is in 1 mol-' cm-'.
-
investigated in this study. The 4p n Rydberg absorption occurs at 209.4 nm. An example of the UPE and the NPE spectra is given in Figure 6. By comparison of the intensities for the two types of spectra, this absorption can be assigned as a parallel transition. This supports the n(bl) (A, A,) Rydberg assignment for the 4p(bl) transition. When the values for the C3coefficients given in Table I11 are used, the polarization angle is determined to be zero. The change in the dipole moment is determined to be (1.9 f 0.2 I D. From the first two coefficents, the change in the polarizability is determined to be -70 f 150 A3, resulting in an excited-state polarizability of 70 f 150 A3. The excited-state dipole moment is determined to be near zero. The 3d n Rydberg absorption occurs at 186.9 nm. Examples of the UPE and NPE are given in Figure 7. From the difference in the intensity between the two spectra, the transition moment is determined to be polarized perpedicular to the major dipole axis, referred to as a "perpendicular"transition. Previous work on ethylene sulfide (thiirane) yielded the same symmetry assignment for the corresponding d Rydberg for the Compound? From the ratio of the C3coefficients for the polarization angle was determined to be 90". The change in the dipole moment was determined to be 14.2 f 0.11 D, twice as large as the value for the p Rydberg. The change in the polarizability was determined to be (420 f 6501 A3 and the excited-state dipole moment was determined to be -2.5 f 0.1 D. This is the second d Rydberg to have been investigated by this method. It is clear in considering the previous data for thiirane that the d Rydbergs are distinct from the other transitions particularly with respect to the excited-state polarizability. These data support the d Rydberg assignments for these bands.
-
+
+
Pentamethylene Sulfide This molecule exists in the chair conformation and belongs to the C, point group. The general features of the spectrum are consistent with the other cyclic sulfides, with
the first p Rydberg absorption being the most intense. The following discussions will be based on the assumption that the molecule has local Cb symmetry for the C-S-C bonds and this local symmetry for the sulfur atom will dominate the absorption characteristics of the compound. The reported photoelectron spectrum for this compound12indicates that the first ionization potential is from a nonbonded orbital, which is also the case for the other cyclic sulfides. In the vacuum-ultraviolet spectrum for this compound, a unique s series, two p series, and several d series of Rydberg absorptions have been identified.12 Assignments were made on the basis of term values for the absorptions since no series leading to the ionization potential was found because of the complexity of the spectrum.12 The origin of the separate p and d series is thought to come from mixing of the p-d orbitals as a result of the low symmetry of the molecule.12 Although it is generally concluded that this mixing is not important in considering the groundstate molecule orbitals, this is not thought to be the situation for the excited-state molecular ~ r b i t a l s . ' ~ JIn ~ Jview ~ of the above difficulties in interpreting the spectrum of PMS, the electric field data were investigated for several reasons: to add data to the cyclic sulfide series; to provide semiquantitative results for the magnitudes of the important properties of the lower excited states; to use these excited-state data in conjunction with the data for the other compounds to aid in the interpretation of the spectrum of the compound. By considering the determined properties in view of the cyclic sulfide series, we noted certain generalities, and they should prove to be useful in elucidating the interpretation for the spectrum of this compound. In the spectrum for PMS given in Figure 3, and bands investigated are labeled according to the assignments given previously.12 In general, the bands were broader than the other bands investigated. Thus, the signal-to-noise ratio was considerably lower. In all cases, fluctuations in the determined coefficients precluded the use of intensity measurements to determine the symmetry of the transitions. The coefficients together with standard errors of the mean at the 95% confidence level (when determined) are given in Table IV. The absorption occurring at 223.7 nm has been identin Rydberg transition. There was no fied as the 4s discernible vibrational structure indicating a geometry change from the chair to the boat conformation. The dipole moment change was determined to be 14.1 f 0.21 D,
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The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 1965
Electric Field Data for Cyclic Sulfides
resulting in an excited-state dipole moment of -2.3 f 0.2
D. The negative value is interpreted as electron density being shifted into the ring in this excited state, resulting in the sulfur atom being the positive end of the dipole. The value for the change in the dipole moment is similar in magnitude as determined for the d Rydberg states found for the other cyclic sulfides. The polarization of the transition was determined to be 90”. This in conjunction with the fact that the polarizability of this excited state is similar to the 4s n transition for thiirane supports the n Rydberg assignment for PMS. 4s There are two absorptions labeled p and p’ occurring at 208.4 and 196.3 nm, respectively. These absorptions have been assigned as the 4p n and 4p’ n Rydberg transitions. These absorptions were assigned by using the term values since a Rydberg series could not be fitted because of the complexity of the spectrum.12 The change in dipole moment for the 4p n Rydberg transition is determined to be 11.61 D. The change in the dipole moment for the 4p’ n Rydberg transition is determined to be 15.71 D. Because of the low single-to-noise ratio for these bands, the uncertainty for the excited-state data was large. This high uncertainty precluded the determination of the polarization angle for these transitions. The dipole moment change for the 4p n Rydberg transition (208.4 nm), however, is consistent with the np assignment with respect to the other cyclic sulfides. The 4p’ n absorption will be discussed in conjunction with the d Rydberg transition below. The 184.3-nm absorption was assigned to the 3d n Rydberg transition. The dipole moment change is determined to be 13.1 f 0.51 D, resulting in a -1.3 f 0.5 D dipole moment for this excited state. The dipole moment change and the polarizability for the excited state are similar to the values determined for the five-member rings. These data support the d Rydberg assignment for this absorption. Sandorfy et al. indicate that the 4p‘ n Rydberg absorption (196.3 nm) and the 3d n Rydberg absorption (184.3 nm) “may correspond to the first members of Rydberg series where the excited state is of mixed p-d character containing an increasing amount of d in this order”.12 These p-d mixtures are possible from the low symmetry of this molecule. The excited-state data for the 4p n Rydberg absorption (208.4 nm) when viewed with respect to the other cyclic sulfides indicate that this assignment is correct. The excited-state data for the 3d n Rydberg absorption (184.3 nm) also supports the trend observed for the cyclic sulfides. It is clear from these data that the 196.3-nm band is distinct from either trend.
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-
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-
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-
-
-
- -
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Discussions and Conclusions The first observation is that the polarization of the transition (parallel or perpendicular) when it could be determined supported the Rydberg assignments. Other assignments involving intravalent transitions which have the same polarizations could not be excluded. However, this polarization information in conjunction with the excited-state data obtained confirms the Rydberg assignments. The dipole moment change (a; pgs- per) is the next excited-state parameter to consider. In all cases investigated, the change in the dipole moment is found to be positive. This sign change in the dipole moment is interpreted as electron density being shifted from the sulfur atom toward the carbon end of the molecule. This is consistent with the originating orbital being localized on the sulfur atom6J1with the change in the dipole moment as a result, then, of the charge delocalization into the ring
structure. This behavior is similar to that observed for the cyclic ketones.2 For the s Rydberg transitions, the dipole moment change for the six-member ring is larger than that observed for the three-member ring. It was observed that the dipole moment change also increased as a function of ring size in the cyclic ketones.2 The polarizability for the sixmember ring is also larger than the corresponding value for the three-member ring. The polarizability for the state is related to the effective size of the molecule orbital; thus the size of the s Rydberg state is larger for the six-member ring than for the three-member ring. The most intense absorption for the cyclic sulfides studied is assigned as the p Rydberg absorption. Data for this absorption are the most complete for the series. As given in Table VI, the change in the dipole moment (a) decreases with an increase in the ring size which is opposite to that observed for the s Rydbergs. In a corresponding manner, the polarizability for this p Rydberg excited state decreases as a function of the ring size. For the d Rydberg transitions, the trend for the change in the dipole moment (a, Table VII) is similar to the p Rydbergs. the change in the dipole moment decreases as a function of the ring size. The polarizability for this state follows a similar trend as observed for the p Rydbergs. The polarization of the transition indicates that the excited state is of B1 symmetry. It is apparent that the d Rydberg states are highly polarizable. Such highly polarizable states are expected to exhibit nonlinearities in the polarizabilities and induced dipole moments as a result of the interaction with an electric field.lg Information concerning this nonlinear behavior cannot be determined from the data presented in this work. The final consideration is the differences observed for the type of transitions. The basic differences among the states is noted when considering the polarizability of the excited state and the magnitude of the change in the dipole moment. In general, these properties indicate the extravalent nature of these states. These properties support the Rydberg assignments in these cases. By considering the absorptions for thiirane (s, p, d) and for T H T (p, d), the ordering for the polarizability of the upper state is s
