2124
J . Phys. Chem. 1988, 92. 2124-2129
On the contrary, the latter model (case b) is such that two different sites exist in the matrix, therefore the two triplet states in different sites are observable simultaneously. This is the most persuasive model to explain our results mentioned so far. The dual phosphorescence phenomena in nonpolar hydrocarbon matrices and in polar solvents have different origins. The thermal equilibrium between adjacent %T* and 3n7r* states coupled by the spin-orbit interaction is responsible for the phenomenon in nonpolar hydrocarbons as reported by Griesser and Bramley.* The b*level is pushed higher in polar solvents than that in nonpolar hydrocarbons; therefore, the spin-orbit interaction between 3na* and 3 7 r ~ *is less effective than that in nonpolar matrices, and thermal distribution between the two states turns out to be difficult. Thus, TB and Tc observed by the time-resolved ESR are two different triplet states in different sites, and probably their conformations are different. TB may have a structure more planar than Tc, and the spin-orbit interaction is weaker than that of T,. The energy diagram based on our results and the available literature is summarized in Figure 8. In this diagram the excited triplet states of benzophenone and hydrated xanthone are employed * 3 ~ 7 r *states, reas typical models of the zeroth-order 3 n ~ and spectively. The principal axes of xanthone are taken as drawn in the insert. The ground state of strongly hydrated xanthone has a planar structure, and the energy separation between 3 ~ 7 r *and is large enough to keep the lowest excited triplet state purely
isolated. This is TAas mentioned before. In the absence of water, the separation between the two states decreases, and hence we expect an effective spin-orbit interaction between them. Recent CIDEP studies of photochemistry of aromatic ketones revealed the reactivity differences of the 3 7 r ~ *and 3 n ~ states * and the solvent effect on the mixing of these states.I6 The zeroth-order levels and the resultant ones (TB and T,) are shown in the diagram. From this diagram, the inversion of the sign and the change of the magnitude of the zero-field splitting parameter, D.in different triplet species are easily understood. In conclusion, xanthone in polar media shows three different triplet states due to the environmental and the conformational differences. In water-containing solvents, a purely isolated ~ r * triplet state is predominant. In dry polar solvents, two triplet states having large D values exist. They are thought to have different conformations, and the SLR rate of a large D component is much higher than that of a small D component. In nonpolar media, only a weak signal of the triplet state having a small D value and showing a distorted spectral shape is detected. The main cause of this spectrum is explained by the energy transfer from the xanthone triplet state having a large D value to the wet xanthone molecule, which attaches a water molecule at its carbonyl group by a hydrogen bond. Registry No. Xanthone, 90-47-1; water, 7732-18-5.
NMR Studfes of Quadrupole Couplings in Dimethyl Sulfone and Carbon Disdflde A. hewenstein* and D. Igner Chemistry Department, Technion-Israel Institute of Technology, Haifa 32000, Israel (Received: August 5, 1987; In Final Form: November 2, 1987)
NMR spectra of dimethyl sulfone (DMS) and carbon disulfide were measured in liquid crystallinesolvents. The order parameters were derived from the proton spectra (DMS) or I3C chemical shifts (CS,). From the observed quadrupolar splittings, the components of the quadrupole coupling tensors of 33Sand 170were derived. Information on the quadrupole coupling constants (QCC) of 33Sand I7O in DMS was also obtained by T I relaxation time measurements in a chloroform solution. The data were supplemented by ab initio calculations of the components of the electric field gradient (EFG) tensors in these molecules. The QCC of 33Sand I7O in DMS are of the order of 1.8 and 8.5 MHz, respectively, while the value for 33Sin CS2 is about 13.8 MHz.
Introduction NMR studies of sulfur-33 and oxygen-17 are relatively Sulfur-33 (spin I = 3 / 2 ) and oxygen-17 (spin I = 5 / 2 ) have low N M R sensitivity and fairly large quadrupole coupling constants (QCC) in many of their compounds (resulting in broad resonances). For these reasons N M R measurements are rather difficult, and special measuring techniques are often advantageo~s.~ Values of the 33Squadrupole coupling tensors are known only in (1) N M R of Newly Accessible Nuclei; Laszlo, P., Ed.; Academic: New York, 1983; Vol. 2, p 388. (2) Vold, R. R.; Sparks, S. W.; Vold, R. L. J . Magn. Reson. 1978, 30,497. ( 3 ) Annunziata, R.; Barbarella, G. Org. Magn. Reson. 1984, 22, 250. (4) Belton, P. S.; Jane Cox, I.; Harris, R. K. J . Chem. SOC.,Faraday Trans. 2 1985, 81, 63. (5) Hakkinen, A.-M.; Ruostesuo, P. Magn. Reson. Chem. 1985, 23, 425. (6) Farrar, T. C.; Trost, B. M.;Tang, S.L.; Springer-Wilson, S. E. J . Am. Chem. S o t . 1985, 107, 262. (7) Ruessink, B. H.; van der Meer, W. J.; MacLean, C . J . Am. Chem. Sot. 1986, 108, 192. (8) Tricot, Y.; Niederberger, W. Helu. Chim. Acta 1984, 67, 1033. (9) Barbarella, G.; Chatgilialoglu, C.; Rossini, S.; Tugnoli, V. J . Magn. Reson. 1986, 70,204.
0022-3654/88/2092-2124$01.50/0
a few simple molecules or molecular fragments from microwave spectra in the gas phase and from ab initio calculations.lO~" Solute molecules in liquid crystals are partially oriented, and consequently their N M R spectra exhibit dipolar or quadrupolar splittings which are not observed in the isotropic media because of complete motional averaging. The quadrupolar splittings (21 lines for spin I) are related to elements of the quadrupole coupling and to order matrix tensors.'* Similar effects are observed when liquid molecules are submitted to very strong electric fields (EF NMR).7 Under favorable circumstances the elements of the QCC tensor for quadrupolar nuclei in small solute molecules can be (10) Lucken, E. A. C. Nuclear Quadrupole Coupling Constants; Academic: New York, 1969; p 287. (1 1) Moccia, R.; Zandomeneghi, M. Advances in Nuclear Quadrupole Resonance; Smith, J. A. S., Ed.; Heyden: Philadelphia, PA, 1975; Vol. 2, p 135. Lucken, E. A. C. Ibid. 1983; Vol. 5 , p 92. Sheridan, J. Ibid. 1983; Vol. 5 , p 145. (12) Emsley, J. W.; Lindon, J. C. N M R Spectroscopy Using Liquid Crystal Solvents; Pergamon: Elmsford, NY, 1975. Khetrapal, C. L.; Kunwar, A. C. N M R Spectroscopy of Molecules Oriented in Liquid Crystalline Solvents, Advances in Liquid Crystals; Brown, G. H., Academic: New York, 1983; Vol. 6, p 173.
0 1988 American Chemical Society
Quadrupole Couplings in DMS and CS2 determined through measurements of their spectra in liquid crystalline solvents.13 The knowledge of the order parameters is essential and usually derived from dipolar splittings. Another procedure to derive QCC's is to measure relaxation times (TI, T2) of the quadrupolar nuclei in isotropic 1 i q ~ i d s . l ~When quadrupolar interactions are the predominant relaxation mechanism (which is usually the case), TI-' or TTl is proportional to the square of QCC and to a reorientational correlation function. The latter may sometimes be derived from relaxation measurements of dipolar nuclei in the same molecule. The present work employs these procedures to sulfur and oxygen in two simple compounds. The difficulties and limitations inherent to these procedures are presented and discussed in detail for each case. Only in two compounds, carbon disulfide2 (CS,) and sulfolane7 (C4HsS02),has a direct experimental determination of the 33S QCC's in the liquid phase been performed by N M R methods. In the first case (CS,) the QCC of 33Swas evaluated from Tl (or T2)measurements of 33S;the reorientational correlation times were derived from TI measurements of I3C. For (C4H8)SO2the E F N M R method, in combination with relaxation time measurements, was applied. The order parameters were obtained from the corresponding ZHmeasurements in the deuteriated molecule, and the analysis of the results was assisted by a b initio calculations of the electric field gradients (EFG) at the 33Snucleus. The I7O QCC in acetone was measured in nematic phases by liquid crystal' and by E F N M R methods.I4 In the present work we report N M R measurements performed on dimethyl sulfone, (CH3),S02 (DMS), and carbon disulfide. Measurements of 'H, 2H, 170,13C,and 33SN M R spectra of these compounds dissolved in liquid crystals and also T Imeasurements of ZH, 170, and 33Sin solutions of DMS in chloroform were performed. Ab initio calculations of the 33Sand 170EFG's have also been carried out to complement the data derived from the experimental measurements. The proton spectrum of DMS in a lyotropic mixture has been reported previously without ana1y~is.I~
Experimental Section Materials. Most compounds used were commercially available and were used without further purification. Deuteriated DMS was prepared by dissolving DMS in basic D 2 0 and extraction (after several days) with benzene. The liquid crystals used were the following: (1) 4-cyano-4'-n-pentylbiphenyl (K- 15 from BDH); (2) Phase IV (Merck); (3) the lyotropic mesophase consisting of a mixture of poly(y-benzyl L-glutamate) (PBLG, molecular weight ca. 130000, obtained from Miles-Yeda) and chloroform; (4) the lyotropic mesophase consisting of sodium decy! sulfate (SdS, obtained from Sigma), decanol, sodium sulfate, and water. Samples. Samples were usually prepared in 10-mm-0.d. N M R tubes. For I3C experiments in CS2, 7.5-mm-0.d. tubes were used which were inserted into 10-mm-0.d. tubes containing deuteriated acetone for locking and external reference. In most experiments the samples were spun. For measurements performed at ambient temperature the stability is estimated to be h0.5 OC while for measurements at higher or lower temperatures the precision is estimated as f l OC. Typical sample compositions were as follows: (a) a saturated solution of DMS in K-15 (the solubility is very low and the exact composition was not determined); (b) 0.06 mL of CS2 in 1 mL of Phase IV; (c) 320 mg of PBLG, 1.6 mL of chloroform (deuteriated or protonated), with either 15-40 mg of DMS (deuteriated or protonated) or 0.06-0.1 mL of CS,; (d) 37.61 wt % SdS, 49.48 wt % D 2 0 , 4.36 wt % decanol, 6.39 wt % Na2S04, 2.16 wt % DMS. N M R Measurements. All measurements were performed on a Bruker AM400WB N M R spectrometer operating at 400.13 M H z for protons. The inversion recovery pulse sequence (r-7(13) hwenstein, A. Advances in Nuclear Quadrupole Resonance; Smith, J. A. C., Ed.; Wiley: New York, 1983;Vol. 5, p 53. (14) Ruessink, B. H.; MacLean, C . Mugn. Reson. Chem. 1987, 25, 365.
2125
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
HERTZ
Figure 1. Proton spectrum of D M S in K-15 (upper trace) together with the simulated (lower trace) spectrum. T = 298 K. Number of transients: 400. X
I
H7
conformotion
I
conformation
n
conformation
m
Figure 2. Three possible conformations for DMS. y axis is perpendicular , symmetry axis for conformations I and 11. to the paper. z axis is the C TABLE I: Geometry of DMS"
s-0 s-c C-H
1.45 1.78 1.09
LOSO
LOSC LCSC LHCH
117 109 103 109.5
'Distances in angstroms and angles in degrees.
r / 2 ) was applied in T1 measurements. Whenever possible the field was locked on one of the two resonances of CDC13 in the ordered phase or on external deuteriated acetone resonance, but the stability of the magnetic field was good enough to perform some of the measurements without lock even for very long acquisition times (about 24 h).
Results Dimethyl Sulfone ( D M S ) . ( a ) Proton Spectrum in K-15. The proton spectrum of DMS in K-15 a t 298 K is shown in Figure 1 (upper trace) together with the simulated spectrum (lower trace) calculated by the PANIC program (a program supplied by Bruker for simulation of spectra). In order to suppress the broad-line background spectrum from the K-15 protons, a delay of 1 ms was applied between the r / 2 pulse and the acquisition. The proton spectrum for a symmetric pair of rotating methyl groups in an ordered phase has been analyzed by Englert et a1.,16 and the assignment is based on the positions of lines marked i, j, k (Figure 1) which depend only on the inter- and intra-methyl dipolar interactions, DAAt and DAA. The value of JAA, is too small (less than 1 Hz) to be evaluated with reasonable precision from the (15) Black, P. J., Lecture delivered at the NMR Meeting in Aachen, Germany, April 1969. (16) Englert, G.; Saupe, A.; Weber, J. P. 2.Naturforsch., A : Astrophys., Phys. Phys. Chem. 1968, 23A, 152.
2126 The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 TABLE 11: Order Parameters for DMS in K-15“ conformation
S,, (XI@)
I I1
-6.07 -5.23 -6.20
111
TABLE 111: Order Parameters for DMS in SdS
S,,, ( ~ 1 0 ~ ) S,, ( ~ 1 0 ~ )
2.87 3.78 3.23
Loewenstein and Igner
conformation
3.20 1.45 2.97
2.29 1.81 2.28
111
I I1 I11
40
20
0 -20 -40 -60 -80 -100 -120
Figure 3. Proton spectrum of DMS in SdS (upper trace) together with the simulated (lower trace) spectrum. Number of transients: 200. T = 305 K .
given spectrum because the line positions cannot be determined accurately enough. This is mostly because the lines are broad (ca. 10 Hz) due to the high viscosity of the solution and the temperature gradient within the sample. The values for the dipolar interaction obtained were DAA = +274.9 f 1.O H z and DAAi = -160.1 f 1.0 Hz. The signs of the D values are opposite but cannot be determined absolutely. The above convention was chosen arbitrarily and conforms with the signs assumed for acetone and dimethyl sulfoxide.”*’s The values of the order parameter can be evaluated from the dipolar interactions, but in order to perform the calculation, the geometry of the molecule must be known (or assumed). The bond distances and angles, taken from X-ray data,19 are given in Table I. Three plausible conformations of the molecule with different orientations of the methyl groups are shown in Figure 2. The inter-methyl dipolar interaction for two methyl groups jumping between the three energy minima can be written as
+ D49 + 0 5 1 + D58 + D59 + O67 + 0 6 8 + D69) (1)
where for conformations I and I1 D69 = D58,D68 = D59,D49 = DS7= D67 = D48and for conformation 111 Dd8= D49rD59= D68, D58 = DS9,D57 = D67. The intra-methyl dipolar interaction for all conformations can be written as DAA =
y3(D45
+ O46 + O56)
(1‘)
where D45 = D46. The D,’s are related to the order parameters in the molecular frame: K D , = 7 ( ~ ~e,s ~, 2 + COS*
S,, (x103)
-3.18 -3.43 -3.16
0.90 1.62 0.88
e2s, + cos2 e,s,)
s,,
(xi031
-1.95 -1.70 -1.80
s,, (xio3)
S,, ( ~ 1 0 ’ )
0.8 1 1.13 0.75
1.14 0.58 1.04
Figure 4. Some typical 33Sspectra in the systems studied. (A) DMS in K-15. Number of transients: 3 X lo6. T = 303 K. (B) DMS in PBLG-CDCI,. Number of transients: 27 350. T = 298 K. (C) CS2 in PBLG-CDC13. Number of transients: 51 800. T = 298 K. A delay
HERTZ
DUf= 1/g(O47 + O48
S,, (x103)
TABLE IV: Order Parameters for DMS in PBLG conformation
60
(xi03)
I I1
’Conformations shown in Figure 2.
loo 80
S ,,
(2)
r?l
rr, is the inter-proton distance and 8, is the angle between the (17) Lindon, J.; Dailey, 8. P. Mol. Phys. 1971, 20, 937. (18) Gopinathan, M. S.;Narasimhan, P. T. J. Magn. Reson. 1912.6, 147.
(19) Sands, D. E. Z . Kristallogr., Krisrallgeom., Kristallphys., Kristallchem. 1963, 119, 245.
of about 80-120 p s was usually applied between the pulse and the beginning of acquisition.
I
I
I
I
I
I
0 -1000 -2000 HERTZ Figure 5. 33Sspectrum of DMS in SdS. The sharp triplet shifted by 4.1 ppm to lower field and As = 1664 Hz is due to the sulfate ion. Number of transients: 90000. T = 305 K. 3000
2000
io00
inter-proton and the molecular x axes, etc.; K = h r 2 / ( 4 r 2 )and s, + s, + s, = 0. Table I1 presents the results for the Sij values obtained from eq 1 and 2 by using the molecular geometry from Table I and the experimentally derived D values. ( b ) Proton Spectrum in SdS. The proton spectrum of DMS in the lyotropic mixture containing SdS together with the spectrum simulated by the PANIC program is shown in Figure 3. The spectrum is not so well-resolved as in K-15 (the order is greater in the latter). Nevertheless, the spectrum contains a sufficient number of well-resolved lines (the viscosity is lower than in K-15) which enables us to determine the dipolar intractions and the spinspin coupling (JU,) with satisfactory precision. The results are DAA= -18.2 f 0.2 Hz, DAN = +5.6 f 0.2 Hz, and J A N = -0.9 f 0.1 Hz. The relative signs are unequivocal, but the absolute signs are arbitrary and were chosen to produce reasonable results for QCC and 7 (described later). The procedure for the determination of the order parameters is analogous to that used for K-15, and the results are given in Table 111. ( c ) Order Parameters in PBLC. We are unable to obtain a clearly resolved proton spectrum in this system and consequently
The Journal of Physical Chemistry, Vol. 92, No. 8. 1988 2127
Quadrupole Couplings in DMS and CS2
TABLE V Quadrupolar Splittings in DMS (Hz)‘ solvent 2H 3 3 s K-15
PBLG SdS
2228 69
170
23280 847 1008
940 2500
T = 298 K except for SdS where T = 305 K.
TABLE VI: OCC Values (in MHz) and n in DMSm
conformation (QCcID,, I I1
Figure 6. I7O spectra of deuteriated DMS in PBLGCDCI3(number of transients: 198000). Note that an I7O quintuplet is observed with splitting of ca. 3100 Hz whose center is shifted ca. 150 ppm to high field.
This is very likely due to the CO groups of the PBLG. A short delay between pulses and acquisitions was applied as in Figure 4.
111
0.178 0.180 0.175
vD (QCC)s,, 0 0 0
1.70 1.92 1.81
(QCC)OSO 0.16 -0.32 0.13
-8.52 -8.71 -8.26
9’
0.16 0.32 0.08
q was assumed 0 for deuterium. (QCC)s and qs were derived from measurements of As in K-15 and SdS (eq 6). ‘(QCC)O and qo were derived from measurements of A. in SdS and PBLG (eq 7).
( e ) Quadrupole Couplings of 2H, 33S,and I7O. The measured quadrupolar splittings A are related to the QCC and q through the relationshipi2
where tl = (VXX- V y y ) / V z z and
QCC = eqVzz/h
In applying eq 3 to our case, we shall make the following assumptions: ( I ) Deuterium. We assume that the quadrupole coupling tensor is axially symmetric (7 = 0) and that its major component is directed along the C-D bond. These assumptions have been verified in many studies of deuterium quadrupole couplings of CD bonds. Under these assumptions eq 1 becomes I
I
3 0 0 2000
I
m
1
0
1
I
I
AD = % ( Q C C ) D S ~ ~
I
- 1 o o o - 2 m -3ooo -4ooo
HERTZ
Figure 7. ” 0 spectrum of DMS in SdS. The single large line shifted 9.4 ppm to lower field is due to the sulfate ion and is not split. The quintuplet (& = 2150 Hz)of water (110 times bigger than the DMS
resonances) is shifted 160 ppm to higher field from DMS and is not shown in the figure. to determine directly the order parameters. The main reasons are the difficulty in extracting the DMS spectrum from the superimposed PBLG and DMS spectra and the small order parameters. Variation of the DMS concentration modifies the PBLG spectrum because the order of the solvent changes, and therefore a simple subtraction procedure could not be applied. The order parameters in this system were derived indirectly by using the measured quadrupolar splitting of 33Sand 2H and values of (QCC)s, vS, and (QCC)Ddetermined in K-15and SdS. The procedure used will be given later. The results of the calculations are shown in Table IV. It may be noted that the order parameters in PBLG are comparable to the corresponding values in SdS and close to those observed for acetone in a similar lyotropic mixture.I7 (d)Deuterium, Sulfur, and Oxygen Spectra. Figures 4-7 show some 33Sand I7Ospectra in the liquid crystalline systems studied. An interesting feature in Figure 7 should be noted: Whereas the I7Oin the D20 and DMS show the expected quintuplet splittings, the I7O of the SO4 ion is a singlet. (The 33Sresonance is split into a triplet; cf. Figure 5.) We have no explanation for this unique behavior. The 170spectrum of DMS in K-15 has not been observed. In this system the splitting between the quintuplet lines is expected to be more than 50 kHz and the line width broader by a factor of -6 than those of 33S.Our A M type spectrometer seems to be unsuitable for such measurements. The quadrupolar splittings (between adjacent lines) for 2H, 33S, and I7O in the different liquid crystals are given in Table V.
(4)
where, for conformation I SCD
= %(COS24 4 J x . x + d4$zz) + 2/3(COS24 5 J X X + cos2 45ysyy + cos2 4 s S z J (5)
4ix,4iy, and 41zare the angles between the C-Hi bond and the x, y , or z axis (cf. Figure 2). Similar expressions can be written
for conformations I1 and 111. ( 2 ) Sulfur. We assume that the major component of the 33S EFG lies along the z axis of the molecule. This assumption, though contradictory to the results obtained by Ruessink et ale7for the similar compound sulfolane and to our ab initio calculation, seems justified as shall be argued later in the Discussion. Hence, eq 3 for 33Sbecomes
( 3 ) Oxygen. It is assumed that the major component of the
I7O EFG lies along the S-0 bond. This assumption is in accordance with the results obtained from the ab initio calculations. The order matrix must be transformed to a coordinate system (x’, y’, z’) where the new z’axis lies along the S-0 bond. The transformation is similar to that described above for deuterium. Equation 1 thus becomes (7)
Each determination of QCC and 7 for a given nucleus requires a pair of A’s measured in two liquid crystals.* The results obtained for the QCC and r) values of 2H, 33S,and 170are given in Table VI. Relaxation Times of DMS in an Isotropic Solution. T,’s of 2H, I7O,and 33Sfor a solution containing 100 mg of deuteriated
2128
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
Loewenstein and Igner
TABLE VII: Ab Initio Calculations for DMS
33SQCC”,d
energy.‘ conformation
hartrees -626.385 69 -626.378 18 -626.382 93
I I1 I11
170
Q C C ~ ~
xx
YY
zz
aa
bb
CC
1.644 1.720 1.554
-3.367 -3.182 -3.292
1.723 1.462 1.738
9.225 9.274 9.243
-4.704 -5.027 -4.889
-4.521 -4.247 -4.354
“(eQ)’ = -6.4 X IO-’’ m2. b ( e Q ) o = -2.6 X mz. CDiagonalizedvalues. The major component lies along the S-0 bond, while the smallest lies in the OS0 plane. +‘(QCC), = ( e Q / h ) V E igiven , in MHz. e l hartree = 627.5 kcal/mol.
DMS in 3 mL of CHCl, were measured. The results are as ) 32 X s; Tl(170) = 5 follows: T1(,H) = 0.72 s; T 1 ( 3 3 S= X 10”s. T I is related to the QCC by the following expression:
In eq 8 it is assumed that reorientation is isotropic and is represented by one correlation time, T ~ .Ruessink et have checked this assumption for sulfolane by comparing the temperature dependence of 33Sand 2H relaxation times. DMS is closer than sulfolane to being a spherical molecule, and consequently we may assume that the anisotropy of the reorientational motion is small. Taking an average value for the three conformations of (QCC)D = 178 KHz and vD = 0, we obtain T , = 2.98 X lo-’’ s. Using this result the QCC’s for I7O and 33Swere calculated by eq 8 with the following results: (QCC)s = 1.63 MHz for 7 = 0 to 1.41 M H z for 17 = 1; (QCC)’ = 8.42 MHz for 17 = 0 to 7.28 MHz for 9 = 1. These results are somewhat lower than those reported above with liquid crystalline solvents (Table VI). The discrepancy may be partly attributed to experimental errors and to solvent effects on the QCC’s. Carbon Disulfide. The 13C and 33Sresonances of CS2dissolved in Phase IV (ca. 0.06 mL of CS, in 1 mL of Phase IV) were measured as a function of the temperature in both ordered and isotropic phases around the transition temperature (ca. 287 K). The extrapolated change in the I3C chemical shift across the transition temperature, referred to an external CO(CH3),, d,was found to be u’
=
u,so- u o r d e r 4
= 14.2 ppm - 12.8 ppm = 1.4 ppm
The 33Squadrupolar splitting, at a temperature just below the isotropic to nematic transition, was As = 33600 Hz. SinceI2 u’
= 2/3AuS,, (for a linear molecule; z lies along the molecular axis) (9)
where Au = u , - ui = 288
+ 143 ppm f 30 ppm = 431 f 30 ppmZ0
and
Combination of (9) and (6’) gives 13.8 f 1.4 MHz as the value for (QCC)s for CS,, which compares well with the value of 14.9 f 0.3 MHz measured by Vold et al., Measurements of the 33Sspectra in PBLG-chloroform gave a triplet with the largest splitting between its components of about 9000 H z at very low CS, concentrations (cf. Figure 4). The splitting decreases when the concentration of CS, increases to the smallest measurable value of about 4000 H z before the solution becomes isotropic. The I3C chemical shifts of CS2 in the same lyotropic system, referred to the CO resonance in external acetone, were also measured as a function of the CS2 concentration. The results for the I3C chemical shifts differences, u’, and As at different CS2 concentrations were as follows: d = 0.42 ppm, As = (20) Pines, A.; Rhim, W. K.; Waugh, J. S. J . Chem. Phys. 1971,54,5438. Spies, H. W.; Schweitzer, D.; Haeberlen, U.; Hausser, K. H. J . M a p . Reson. 1971, 5, 101.
7700 Hz; u’ = 0.39 ppm, As = 8930 Hz; u’ = 0.22 ppm, As = 6000 Hz. Using eq 9 and 6’, we can calculate values for (QCC)s. The average result is (QCC)s = 13.2 f 3.0 MHz. The large error in this measurement is probably a consequence of the small absolute magnitude of u’ and its variation with CS, concentrations. The correction of u’ due to the susceptibility difference between the isotropic and nematic phases was estimated from literature data and found to be negligible. Ab Initio Calculations. The energies and the electrostatic properties (EFG’s) were calculated by employing the GAUSSIAN 82 program at the 6-3 1 G* level.” Similar calculations for the DMS anion were performed by Bors and Streitwieser, Jr.22 The results for DMS are given in Table VII. The ab initio calculations also gave a dipole moment of about 5.5 D (compared to the experimental value 4.5 D) directed along the z axis. The auerage major component of (QCC)Dcalculated was 195 KHz, with 9 close to zero, but large variations of the individual EFG’s were noted. The calculated values for the components of the EFG tensor in CS2 are (QCC),, = 20.51 MHz and 9 = 0 (aa lies along the molecular axis).
Discussion One of the problems in the interpretation of the experimental results for DMS is the nonrigidity of the molecule, Le., the fact the methyl groups may jump between different conformations. Although our a b initio calculations indicate that the energy of conformation I is somewhat lower than those of conformations I11 and particularly 11, it should be noted that (a) these calculations are performed for the isolated molecule, not taking into account the solventsolute interaction energy which, especially in complex solvents as liquid crystals, might contribute significantly to the total energy and hence determine its most stable conformation, and (b) an ab initio search for the optimal conformation with lowest energy has not been performed. Another possible source of error is the use of two different liquid crystalline systems for the evaluation of the QCC’s and 9’s. It is well-known that the quadrupole coupling constants, especially for peripheral atoms with low QCC values, may be sensitive to the solventsolute interaction. This poses a very difficult problem when attempts are made to measure the QCC’s with high precision. The ab initio calculated directions of the major components of the EFGs in 170and deuterium agree well with the experiment, the calculated values of the QCC’s being about 10% higher than the experimental values. For 33Sthis does not seem to be the case. The largest component of the calculated EFG lies along the molecular y axis while the experiment shows clearly that it must lie along the molecular z axis. Considerations of orbital charge density distribution and the direction of the molecular dipole moment support this conclusion. The magnitude of the calculated major component (ca. 3.2 MHz) by far exceeds the measured values (ca. 1.8 MHz). Ruessink et al.7 obtained similar results in their ab initio calculations for the 33SQCC elements in the similar molecule of sulfolane with ( e Q / h )Vyy(in their notation Vbb)= +3.55 MHz. They assumed that their maximal measured component (1.23 MHz) also lies along the y axis. On the basis (21) We are indebted to Dr. M. Karni for performing these calculations and helpful discussions and to Prof. Y . Apeloig for his assistance. Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y . ; Karni, M.; Chandrasekar, J.; Schleyer, P. v. R. J . Am. Chem. SOC.1986, 108, 270. (22) Bors, D. A.; Streitwieser, A., Jr. J . Am. Chem. SOC.1986, 108, 1397.
J. Phys. Chem. 1988, 92, 2129-2133 of our experimental results and assuming that the electrostatic properties of its sulfur are similar to those in DMS, we doubt their assignment. The a b initio calculations for the electrostatic properties of 33Sby the GAUSSIAN 82 program seem to be unreliable, contrary to the results for 170or 2H. The reason for this is not quite clear to us but may be related to the fact that the charge densities are derived by the Mulliken population analysis.21 Our results, in general, seem to give acceptable values for the QCC's. The experimental precision of the QCC values in DMS in each conformation is estimated to be zkS% and could be improved if the order parameters and quadrupolar splittings would be determined with higher precision. The results for the values of 7 are much less precise, and the cause can be traced to the small numerical differences between the elements of the order matrix. The value of 7 (eq 3) is very sensitive to these differences.
2129
For CS2 our results conform quite well to those obtained by relaxation methodsZwhile the calculated ab initio values are again too high. The high symmetry and rigidity of this molecule facilitate the interpretation, though the procedure of obtaining the order parameters from the 13Cshifts is inferior to that of deriving them from proton spectra and causes relatively large errors in the determination of the QCC's. The reason is the small, but not negligible, temperature or concentration differences that exist between the measurements in the isotropic and the ordered solutions. Acknowledgment. This research was supported by the Fund for the Promotion of Research at the Technion. Registry No. DMS, 67-71-0; CS2, 75-15-0; 33S,14257-58-0; "0, 13968-48-4.
Direct Observation of Twisted Excited Triplet States of Monocyclic Conjugated Enones by Tlme-Resolved Electron Paramagnetic Resonance Seigo Yamauchi,* Noboru Hirota,* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
and Jiro Higuchi* Department of Chemistry, Faculty of Engineering, Yokohama National University, Yokohama 240, Japan (Received: August 10, 1987; In Final Form: October 21, 1987)
We have made time-resolved EPR (TREPR) studies on the lowest excited triplet states of a series of flexible monocyclic conjugated enones in various solvents at 77 K. TREPR spectra were obtained for 2-cyclopentenone (CPe), 2-cyclohexenone (CHx), and 1-acetylcyclohexene (AcCHx) in trifluoroethanol, ethanol, and methylcyclohexane, in which very little solvent effects were observed. A systematic decrease in zero-field splittings (zfs) was found for CPe, CHx, and AcCHx in this order in accordance with rigidity of the molecules. The time profile of the TREPR signals showed polarization inversion. From the analyses of the decay curves it was found that the triplet lifetimes also decreased in the same order as in the case of the zfs. The zf sublevel schemes are determined on the basis of the magnetophotoselection spectra and a theoretical consideration for the SI T I intersystem crossing rates. From the magnitudes of the zfs, the zf schemes, and the lack of the solvent effect * From a simple calculation for the zfs based on the Hiickel MO's on the zfs T I is assigned to be nearly pure 3 ~ in ~nature. and a half point-charge approximation the decrease in the zfs is explained in terms of the distortions (rotations around the C=C bond) of the molecules. A strong correlation between the magnitude of the distortion and the triplet lifetimes leads to a conclusion that the distortion is the main cause for the nonphosphorescent nature of these partially flexible enones.
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1. Introduction Excited states of various kinds of alkenes are considered to be stable in twisted forms around ethylenic C=C double bonds. Ethylene,' stilbene,2 and cycloalkenes3 are such molecules which have been studied extensively by means of a variety of experimental techniques and theoretical calculations. Theoretical studies have suggested that the stable structures of the excited states are twisted ne^.^^.^-^ In contrast it is rather difficult to obtain direct evidence for the twist of the C = C bond from experiments. Since this type of molecules does not emit, it is not possible to obtain evidence for the twisted structures from the emission and excitation spectra. The lack of the emission itself seems to present indirect evidence for the twisted structure^,'-^ but it is not definitive. A transient (1) Merer, A. J.; Mulliken, R. S . Chem. Reu. 1969, 69, 639. (2) (a) Hammond, G. S.; Saltiel, J.; Lamola, A. A,; Turro, N. J.; Brad-
shaw, J. s.;Cowan, R. C.; Counsell, R. C.; Vogh, V.; D a h , C. J . Am. Chem. Soc. 1964, 86, 3197. (b) Jortner, J.; Rice, S. A,; Hochstrasser, R. M. Adu. Photochem. 1969, 7, 149. (3) (na) Zimmerman, H. E.; Kamm, K. S.; Werthemann, D. P. J . Am. Chem. Soc. 1975, 97, 3781. (b) Brumi, M. C.; Momicchioli, F.; Baraldi, I. Chem. Phys. Lett. 1975, 36, 484. (4) (a) Becker, R. S.; Inuzuka, K.; King, J. J . Chem. Phys. 1970,52,5164. (b) Wagner, D. J.; Schere, B. J. J. Am. Chem. SOC.1977,99, 2888. (c) Birge, R. R.; Leermarkers, P. A. J. Am. Chem. SOC.1972, 97, 8105. (5) Devaquet, A. J . Am. Chem. SOC.1972, 94, 5160.
0022-3654/88/2092-2129$01.50/0
absorption technique is a suitable method to study the properties of the excited states, and transient absorption spectra as well as decay rate constants were obtained in several systems.6~~Although the results were suggestive of the twist, these are again weak as the experimental evidence for involvement of the rotation around the C=C bond. An EPR technique is useful for studying the structures of the lowest excited triplet (T,) states. In this case, however, neither conventional EPR nor optically detected magnetic resonance (ODMR) techniques can be applied because of the very short lifetimes (e1 ms) and nonphosphorescent character of the T, states. A time-resolved EPR (TREPR) technique has been shown to be very effective in studying such short-lived nonphosphorescent triplet states.8 With this technique we can obtain the magnitude of zero-field splittings (zfs) and lifetimes of the T1 states. As the observed zfs are directly related to the geometry of the T1state, one can discuss the Tl structure in connection with the rotation around the C=C bond. We can further examine a correlation between the zfs (structures) and the lifetimes, which may be (6) Goldfarb, T. J . Photochem. 1978, 8, 29. (7) Bonneau, R. J . Am. Chem. SOC.1980, 102, 3816. (8) (a) Terazima, M.; Yamauchi, S.; Hirota, N. J. Phys. Chem. 1985, 89, 1220. J . Chem. Phys. 1986.84, 3679. (b) Yamauchi, S.; Hirota, N. J . Chem. Phys. 1987, 86, 5963.
0 1988 American Chemical Society
