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Experimental and theoretical studies of isomeric C2H5S and C2H5S+
Volume 2 I 3, number 3,4
CHEMICAL PHYSICS LETTERS
8 October 1993
Experimental and theoretical studies of isomeric C2HSS and C,H$+ Z.-X. Ma, C.-L. Liao, H.-M. Yin i, C.Y. Ng Ames Laborarory ‘, USDOE, Ames, IA 5001 I, USA and Department of Chemistry, Iowa State University,Ames, IA 50011, USA
See-Wing
Chiu
Biotechnology Center, Universityofllinois, Urbana, IL 61801, USA and National Center for Supercomputing Applications, Universityof Illinois. Champaign, IL 6 1820, USA
Ngai Ling Ma and Wai-Kee Li Department of Chemistry, The Chinese Universityof Hong Kong, Shatin. N. T:. Hong Kong
Received I1 June 1993
By combining photoionization and photodissociation measurements with ab initio Gaussian-Z (G2) calculationson the CZHSS and C2H$+ system, we have concluded that CHsCH2Sis the dominant primary product formed in the 193nm photodissociation of (CH,CH&S, while CH,CHSH+ is the product ion formed at the photoionization onset of CzH$+ from (CHICHZM The G2 predictions for the beats of formation at 0 K (A&) for theisomers of C2HISand &HIS+ (CHKH2S: A&(02 )=27.5kW mokCH,SCH,: A&,,(G2)=37.3 keal/mol; CH,CH,S+:A&(G2)=236.5 kcal/mol;CH,CHSH+:A&(G2)= 192.6kcaUmol; and CH$CH; : A&,(G?) = 195.3 kcal/mol) are in agreement with available experimental AHm values (CHFHzS: AFIm(exp)=31.4f2kcal/mol; CH,SCH2: AHr0/(exp)=34.8f2.5keal/mol; CH&HZF: ~,(exg)=238.3+2kcal/mol; CH&HSH+: A&,(exp)= 189.6+ 1.0 kcal/mol; and CH$CH:: A&,(exp) = 195.1kcat/mol). The G2 calculationabe yields a value of 35.3 kcal/mol for LI&,,(CH,CHSH).
1. Introduction The characterization of the energetics for isomers of polyatomic radicals and their ions is a great challenge to both experiment and theory. According to ab initio calculations [ 1] using the second-order Moller-Plesset (MP2) perturbation theory with the 6-3 lG( d) basis set [ 2,3] CzH,S may consist of at least four stable isomers: CH,CH,S (I), cis-CH,CHSH (II), tram-CH,CHSH (II’) and CH,SCH, (III) s’. The corresponding isomers for
’ Visiting scientist. Permanent address: Dahan Institute of Chemical Physics, Dalian, People’s Republic of China. 2 Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. W-7405-Eng82. This article was supported by the Division of Chemical Sciences, Offme of Basic Energy Sciences.
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&H,S+ are CH3CH2Sf (I+), cis-CH&HSH+ (II+ ), trans-CH,CHSH+ (II’+ ), and CH&CH: (III+ ) (footnote 1). Based on the thermochemistry reference book compiled by Lias et al. [ 41 the heats of formation at 0 K (mfo) for these species are not known, except those for II+ (or II’+) and III+, where the AH,-,, values have been estimated from appearance energy (AE) measurements. One of the most important aspects of a photochemical experiment is to identify the structures of primary photoproducts and the branching ratios of primary product channels. When polyatomic radicals and radical ions formed in photodissociation (PD) or photoionization (PI), respectively, are de#* The calculation (ref. [ 1 ] ) indicates that CH$JHSH and CH&HSH* consist of the cis- and tram-conformers with the H attached to S cis and bans to the CH3 group, respectively.
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 213, number 3,4
CHEMICAL PHYSICS LETTERS
tected by mass spectrometry, their structures usually cannot be identified if the energetics for the poly atomic radicals or radical ions of interest are not known. In such cases, the combination of experimental measurements and reliable theoretical predictions is desirable for identifying the isomeric structures of primary photofragments. Here, we present the results of a combined experimental and theoretical study of &H,S and C2HSS+ formed in the PD and PI processes, respectively, CH3CH2SCH2CH3+hv (193 nm) -KzH$ + CzHs ,
(1)
CH3CH2SCH2CHl+h~+C2H$++C2Hg.
(2)
The CZHSSradicals produced by process ( 1) have also been studied by PI mass spectrometry according to C,H,S+ hv-C,H,S++e-
.
(3)
Quantum-chemical calculations for the CzH$Sand CLH$+ system have been made using the ab initio Gaussian-2 theoretical procedure [5-lo]. Previous investigations indicate that the G2 predictions for molecular energies, such as ionization energies (IEs ) and heats of formation (A&,,), for organosulfur molecules, neutrals and ions, are accurate to * 0.15 eV [ 7-91. After comparing the measured IE for process (3) and the AZ&, value for C,H,S+ formed in process (2) with G2 theoretical predictions, we have assigned structures for C2H,S and CzH,S+ observed in the PD and PI processes ( l)(3). When experimental IEs and A&, values for isomerit C,H,S and &H&S+ are available [ 1l-l 3 1, we find good agreement between the experimental results and the G2 theoretical predictions.
8 October 1993
seeded in Ar is produced by expansion through a pulsed valve with a nozzle diameter of 0.5 mm and a total stagnation pressure of 1150 Torr. The pulsed valve is operated at 40 Hz. The gas beam is skimmed by a conical skimmer before entering the PI region of a quadrupole mass spectrometer_ In this experiment, a supersonic beam of C2H5S radicals is prepared by 193 nm laser PD of a ( CH&H2)zS pulsed jet. The PD ArF excimer laser beam is mildly focused and intersects the ( CH,CH2).$ free jet at 90” and x 2 mm from the nozzle tip. The triggering pulse for firing the excimer laser is delayed by 500 ns with respect to the triggering pulse for opening the pulse valve. The excimer laser pulse energies used vary in the range of 35-80 mJ. Previous studies support that the internal excitations acquired by the C2HSSradicals in the PD process are relaxed efficiently as the radicals entrained in the carrier gas flow expand further into the vacuum. Photoionization sampling of the radical beam takes place xlcm from the nozzle tip. The &H&S+ ion beam pulse, observed by the PI of CzH$ formed in the PD radical supersonic source using a multichannel scaler, has a full-width-at-half-maximum (fwhm) of z 800 us [ 16,171. The C,H,S+ ion counts are measured by gating the scaler within the temporal range of 0.9-2.9 ms with respect to the triggering pulse for opening the pulse valve. The typical counting time for C,H,S+ from CzHSS at each PI wavelength is 5 min The same experimental procedures are used in the PI measurements for process (2) except that the PD excimer laser is turned off and the counting time at each wavelength is typically 15-20 s. The PI wavelength resolution used is %1.5 8, (fwhm ) . 2.2. Theoretical calculations
2. Experimental and theoretical methods 2.1. Experiment The PI study of processes (2 ) and (3) has been performed using the molecular beam PI mass speG trometer [ 14- 171. The experimental procedures used are similar to those described in the recent PI studies of CS, SO, and CH$ [ 16,173. The pulsed jet of (CH,CH,),S (vapor pressure M19OTorr at 323 K)
The ab initio G2 theoretical procedure has been described in detail by Curtiss et al. [ $61. Briefly, at the G2 level of theory, molecular structures are optimized using the MP2 perturbation calculations with the 6-31G’ basis set and with all the electrons included (MP2 (full) /6-3 1G*). AI1 single-point calculations involved are based on the MP2/6-31G* optimized structures. The G2 method, an approximation of a QCISD(T)/6-311 +G(3df, 2p) calculation, requires single-point calculations at the MP4/ 251
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6-31 lG”, MP4/6-311 +G”, MP4/6-31 lG(2df, p), QCISD(T)/6-311G”,andMP2/6-311+G(3df,2p) levels. A small semi-empirical correction is applied to account for the high level correlation effects. The MP2/6-3 1G’ harmonic vibrational frequencies, scaled by 0.93, are used for correction of zero-point vibrational energies (ZPVE) [ 91. The total energy at 0 K (Eo) is equal to E, +ZPVE, where E, is the electronic total energy. All single-point calculations have been carried out on CKAY-YMP and CRAY-2 using the GAUSSIAN 90 and GAUSSIAN 92 package programs [ 23 ] _
3. Results and discussion The PI efficiency (PIE) spectra for C2HSS+from CzHSS (process (3) ) and for CzH$+ from (CH,CH*)$ (process (2) ) are depicted in figs. la and lb, respectively. The PIE spectrum for (CH,CH&S (not shown here) has also been measured. The IE observed for (CHSCH2)zS (8.41 fO.O1 eV) is consistent with the literature value of 8.43 + 0.0 1 eV [ 41. The ionization threshold for CzHsS formed in process (1) is sharp with an IE of 8.97 + 0.0 1 eV ( 1382 + 2 A), indicating that the structure for C,H,S+ resulting from process (3) is similar to that for C2H& The previous photofragmentation studies of organosulfur molecules at 193 nm suggest that the dominant dissociation pathways involve cleavage of the C-S and S-H bonds
10
(b) 8
4
C&S+/CeHSSC,H,
r(a' */CH,CH,S CH,CH,S
Fig. I. PIE spectra: (a) CH$H$* formed in the PI of CH,CH# in the wavelength range of 1351%14OOA and (b) CIH,S+ formed intheP1 of (CH,CH,),S in the wavelengthrangeof 1150-1270A.
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8 October1993
[ lo-12,16,17]. Thus, the C2HSSproduced in process ( 1) most likely has the I structure and the ionization of I should yield I+. The sharp ionization onset for I is consistent with the fact that the ionization of this radical involves the removal a nonbonding electron from S [ 181. The fact that the IE for I is slightly lower than that (9.23+0.01 eV) [ 161 for CH$ can be attributed to the greater inductive effect for CH,CHZ compared to CHj. The AE for process (2) is determined to be 12232 5 A ( 10.13 20.04 eV), the energy value at the intersection of the sharp rise in PIE for CzH$+ and the background baseline, as indicated in fig. lb. The uncertainties of 5 5 A account for the finite PIES observed in the range of z 1220-1228A. Since the (C2H5)$ sample is introduced as a pulsed supersonic beam, we assume that the hot band effect is not important. Using 1224f 5 A as the AE for C,H,S+ from (CH3CH2)$, and the known values for A% [ ( CI-W% )3 1 and A&o(CI-WM 14I, the AHfovalue for C,H,S+ formed in process (2 ) is determined to be 189.6kcal/mol. Previous studies show that the mass 47 ions observed from CH,SCH, and from CHaSH have the CH,SH+ structure at their PI onsets [ 7,9,19]. Similarly, the C2HsS+ ions formed in process (2 ) are most likely II+ or II’+. In order to confirm the speculation about the assignment of I, II+ (or II”), and I+ for CzH$ and C2H5S+ formed in processes (1 ), (2), and (3), respectively, G2 calculations have been made for I, II, II’, I+, II+, and II’+. Since the energetics for III and III+ have been determined in recent experiments [ 1 1- 131, we have also performed calculations for this CzH$ isomer and its ion. Figs. 2a-2d show the structures for I and I+, II and II+, II’ and II’+, and III and III+, respectively, optimized at the MP2/63 1G* level. As expected, the structures for I and I+ are very similar and both have a C, symmetry. The ionization of I in the ground ‘A” state yields I+ in the ground 3Anstate. The removal of an nonbonding electron from S has the effect of shortening the C-S bond by 0.037 A with concomitant lengthening of the C-C bond by 0.02 A. Both the ‘A ground states for II and II’ have a C, symmetry. The ionization of II and II’ involves the ejection of the unpaired electron from the middle C atom. As a result, the bonding around the C atom is transformed from a pyramidal to a planar structure and the structures for II+ and
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CHEMICAL PHYSICS LETTERS
II’+ in the ‘A’ ground states have a C, symmetry. Similarly, the ground state (‘A’) for III+ also results from the ionization of the unpaired electron from the C atom of the CH, group in III. Table 1 lists the G2 theoretical values for Er,, IE, and AZ&,of I, II (II’), III, I+, II+ (II’+), and III+. We find that the G2 IE (IE(G2)) value of 9.07 eV for I is in good agreement with that observed for CzHSSformed in process ( 1), supporting the expectation that the CzHSSradicals formed in the 193 nm PD of (CHsC!Hz)# have the I structure. The IE(G2) values for the other neutral isomers are less than 7 eV. The observation that the PIES for C2HSS+ are neg-
ligible below the IE for I can be taken as evidence that I is overwhelmingly the dominant photoproduct, with negligible contribution from isomers such as II (or II’) and/or III. Based on the energy release measurement of the process, CH&H$H+ hu( 193 nm)-&H,S+H, and assuming that CzHSS produced has the I structure, A&(I) is deduced to be 31.4+ 2 kcal/mol [ 171, This value, determined directly from the kinetic energy release measurement, agrees with the literature value of 33 kcal/mol determined by kinetic methods [ 2 11, These values for m,(I) are higher than the G2 value of 27.5 kcal/ mol for mro(I) by o4kcal/mol. Using Mro(I)
a
b .
&l-2-3 = -32.1
-I L6-3-2-1
105.8
=56.0 = -154.0
L4-3-2-l = 170.1
1
+
105.7 107.9
= 122.9
= 60.7
(I’) c, Fig. 2. Geometric structures: (a) CH$H$(C,; CH,CHSH+(C,; ‘A’); (c) tram-CH&HSH(C,;‘A) CH,SCH: (C,; ‘A’) (III+).
(II+)c, ‘A”) (I+); (b) cis-CH,CHSH(C,; zA) (II) and cis2A”) (I) and CH,CH$+(C.; (II’) andtrans-CH,CHSH+(C,; ‘A’) (II’+); and (d) CH$CH2(C,;‘A) (III) and
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CHEMICALPHYSICSLETTERS
Volume 213, number 3,4
I
L4-l-2-3
L6-3-2-l
=-177.8
I44.4 = 179.6
L4-3-2-i =164.0
(III)
c,
L43-2-1 = 122.8
(II’+) c,
(III’) c, Fig. 2. Continued.
=31.422 kcal/mol and the IE for I+I+ obtained here, we calculate a value of 238.3+2 kcal/mol for A&(1+), which is in excellent agreement with the G2 value of 236.5 kcal/mol. The cis- and trans-isomers, II+ and II’+, are predicted by the G2 theory to be more stable than I+ and III+. The AZ&Ovalues obtained by G2 calculations for II and II’ are essentially identical as are the values for II+ and II’+, Based on the comparison of G2 predictions with the experimental Mfo value of 189.6 + 1.Okcal/mol for CzHSS+ formed in process (2), we have assigned II+ (or II’+) as the structure for C&S+ formed in process (2) N2. A value xz The G2 calculation shows the AHmvalues for II’+ and II+ are essentially identical. Thus, C2HIS+ formed at the AE from (CH,CH,)$ may have the II+ and/or II’+ structures.
254
for recommended 200 kcal/mol is of Lw, (CH$HSH+ ) in the most recent thermochemistry compilation [ 41. This value, based on the AE for C,H,S+ from the PI of CH$SCH3 measured by Baer and co-workers [ 201, is significantly greater than the value for A&,(11+ or II’+) determined here. In the original reference of Baer and co-workers, the structure for the observed C2H$+ from CH$SCH, is not assigned. Instead, III+ is suggested as the possible ion based on collisional activation studies and ab initio calculations [ 201. The experimental A&(111) =34.8 k2.5 kcal/mol and A&(111+) = 195.1 kcal/mol determined in a recent PI and PD study [ 121 are consistent with the value A&,JIII) = 35.3 5 1.6 kcal/mol obtained in a recent kinetic measurement [ 13 ] and the literature value A&( III+) =
CHEMICAL PHYSICS LETTERS
Volume 2 13, number 3,4
8 October 1993
Table I Comparison of experimental values and theoretical predictions for the IE and A&c, of CH,CH,S(C,; 2A”) (I), cis-CH&HSH(Ci; 2A) (II), trans-CH$ZHSH(C,; ‘A) (II’), CHsSCH2(C1; ‘A) (III), CH,CH,S+(C,; ‘A”) (I+), cis-CH,CHSH+(C,; ‘A’) (II’), transCH,CHSH+(C,; ‘A’) (II’+), andCH,SCH: (C,; ‘A’) (III+) Species
Eo(G2) (hartree)
A&,,(GZ) ‘) (kcal/mol)
CH,CH#( C.; ‘A”) cis-CH,CHSH(Ci; ‘A) tram-CH,CHSH (C,; zA) CH,SCHI( C,; ‘A)
-476.73761 -476.72512 -476.72459 -476.72189
27.5 35.3 35.6 37.3
CH,CH2S+ (C,; 3A”) cis-CH&HSH* (C . ‘A’) trans-CH,CHSH+ ;‘C.; IA’) CH,SCH; (C tr. ‘A’)
-476.40417 -476.47441 -476.47427 -476.46987
236.5 192.6 192.6 195.3
uf0(w
)
(kcal/mol) 31.4?2
34.8L2.5 =) 35.3k1.6”’ 238.3*2 189.6f 1.0”
IE(G2) b’ (eV)
IE(exp) b, (eV)
9.07 6.82 6.81 6.85
8.91 fO.O1
6.94 =)
1
195.1 c) 200 ‘) 194s)
‘I Calculated using the Eo(G2) values for species shown in the table, the Eo(G2) values for S, C, and H given in ref. [ 91, and the A& values for S (65.6 kcal/mol), C (170.0kcal/mol), and H (51.63 kcal/mol) given in ref. [4]. b, The IE for I, II, II’, and III correspond to the formation of I+, II+,II'+, and III+, respectively. “Ref. [ll]. d)Ref. [13]. ‘1 Calculated using the A&(exp) values for III and III+. f, The experimental value may be assigned to A&( II+ ) or aH, (II’+ ) . I) Refs. [4,20], This value is recommended as AJfm(CHICHSH*) by Lias et al. (41. In the original reference (ref. [20] ), III+ is suggested as the structure for C,H,S+ formed at the AE in the dissociative photoionization of CH$SCHs (see text). s) Ref. [4].
194 kcal/mol [ 41. Since the mfo value of 200 kcal/ mol obtained by Baer and co-workers [ 20 ] is closer to AJYHfo(III+) than to A&O(II‘c or II”), it is more likely that the CzH5S+ ion observed in the PI of CH$SCH, corresponds to III+. Combining the experimental M, values for III and III+ yields a value of 6.94 eV for the IE of III.+III+, which is in good agreement with the G2 IE for III. As shown in table 1, the deviations between the G2 and experimental values for m,(I), m,(I+), A&(111), and mH,(III+ ) are less than 4 kcal/mol. In summary, the comparison between the experimental and theoretical IE and L&~ values shown in table 1 supports the conclusion that I is formed in the 193 nm PD of (CH&H2)zS and that the PI of I gives I+ at the ionization threshold. Furthermore, the C2H5S+ ions observed at the AE in the PI of (CH$Hz)$ most likely have the II+ (or II’+) structure. The utilization of high-level ab initio predictions for molecular energies is highly profitable for identifying the isomeric structures of primary polyatomic photoproducts.
Acknowledgement CYN acknowledges the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. NLM and WKL wish to thank the support of a Hong Kong University and Polytechnic Grants Committee Earmarked Grant for research (Account No. 221600080).
References [ 1] S.-W, Chiu, N.-L. Ma, W.-K. Li and C.Y. Ng, to be published. [2 ] W.J. Hehre, L. Radom, P von R. Schleyer and J.A. Pople, Ab initio molecular orbital theory (Wiley, New York, 1986). [ 3 ] M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Raghavachari, M.A. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrccs, D.J. Fox, R.A. Whiteside, R. Seeger, CF. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and JA Pople, GAUSSIAN 90 (Gaussian Inc., Pittsburgh, PA 15213, 1990).
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[4] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17, Suppl. No. 1 (1988). [ 51L.A. Curtiss, K. Raghavachari, G.W. Trucks and J.A. Pople, J. Chem. Phys. 94 (1991) 7221. [ 61 L.A. Curtiss, L.D. Kock and J.A. Pople, J. Chem. Phys. 95 (1991) 4040. [ 71 R.H. Nobes and L. Radom, Chem. Phys. Letters 189 ( 1992) 554. [R]L.A. Curtiss, R.H. Nobes, J.A. Pople and L. Radom, J. Chem. Phys. 97 (1992) 6766. [9] S.-W. Chiu, W.-K. Li, W.-B. Tzeng and C.Y. Ng, J. Chem. Phys. 97 (1992) 6557. [lo] S. Nourbakhsh, C.-L. Liao and C.Y. Ng, J. Chem. Phys. 92 (1990) 6587. [ 111S. Nourbakhsh, K. Norwood, H.-M. Yin, C.-L. Liao and C.Y. Ng, J. Chem. Phys. 95 (1991) 946,5014.
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[ 121% Nourbakhsh, H.-M. Yin, C.-L. Liao and C.Y. Ng, Chem. Phys. Letters 183 (1991) 348; 190 (1992) 469. [ 131P.H. Wine, private communication. [ 141Y. Ono, S.H. Linn, H.F. Prest and C.Y. Ng, J. Chem. Phys. 73 (1980) 2523. [ 151C.Y. Ng, Advan. Chem. Phys. 52 ( 1983) 265. [ 161 S. Nourbakhsh, K. Norwood, G.-Z. He and C.Y. Ng, J. Am. Chem. Sot. 113 ( 1991) 63 11, and references therein. [ 171 K. Norwood, S. Nourbakhsh, G.-Z. He and C.Y. Ng, Chem. Phys. Letters 184 (1991) 147. [ 181 C.-W. Hsu, C-L. Liao, Z.-X. Ma, P.J.H. Tjossem and C.Y. Ng, J. Chem. Phys. 97 (1992) 6283. [ 191 B. RuS&?and J. Berkowitz, J. Chem. Phys. 97 ( 1992) 1818. [20] J.J. Butler, T. Baer and S.A. Evans Jr., J. Am. Chem. Sot. 105 (1983) 3451. [2 I ] D.F. McMillen and D.M. Golden, Ann. Rev. Phys. Chem. 33 (1982) 493.
CHEMICAL PHYSICS LETTERS
8 October 1993
Experimental and theoretical studies of isomeric C2HSS and C,H$+ Z.-X. Ma, C.-L. Liao, H.-M. Yin i, C.Y. Ng Ames Laborarory ‘, USDOE, Ames, IA 5001 I, USA and Department of Chemistry, Iowa State University,Ames, IA 50011, USA
See-Wing
Chiu
Biotechnology Center, Universityofllinois, Urbana, IL 61801, USA and National Center for Supercomputing Applications, Universityof Illinois. Champaign, IL 6 1820, USA
Ngai Ling Ma and Wai-Kee Li Department of Chemistry, The Chinese Universityof Hong Kong, Shatin. N. T:. Hong Kong
Received I1 June 1993
By combining photoionization and photodissociation measurements with ab initio Gaussian-Z (G2) calculationson the CZHSS and C2H$+ system, we have concluded that CHsCH2Sis the dominant primary product formed in the 193nm photodissociation of (CH,CH&S, while CH,CHSH+ is the product ion formed at the photoionization onset of CzH$+ from (CHICHZM The G2 predictions for the beats of formation at 0 K (A&) for theisomers of C2HISand &HIS+ (CHKH2S: A&(02 )=27.5kW mokCH,SCH,: A&,,(G2)=37.3 keal/mol; CH,CH,S+:A&(G2)=236.5 kcal/mol;CH,CHSH+:A&(G2)= 192.6kcaUmol; and CH$CH; : A&,(G?) = 195.3 kcal/mol) are in agreement with available experimental AHm values (CHFHzS: AFIm(exp)=31.4f2kcal/mol; CH,SCH2: AHr0/(exp)=34.8f2.5keal/mol; CH&HZF: ~,(exg)=238.3+2kcal/mol; CH&HSH+: A&,(exp)= 189.6+ 1.0 kcal/mol; and CH$CH:: A&,(exp) = 195.1kcat/mol). The G2 calculationabe yields a value of 35.3 kcal/mol for LI&,,(CH,CHSH).
1. Introduction The characterization of the energetics for isomers of polyatomic radicals and their ions is a great challenge to both experiment and theory. According to ab initio calculations [ 1] using the second-order Moller-Plesset (MP2) perturbation theory with the 6-3 lG( d) basis set [ 2,3] CzH,S may consist of at least four stable isomers: CH,CH,S (I), cis-CH,CHSH (II), tram-CH,CHSH (II’) and CH,SCH, (III) s’. The corresponding isomers for
’ Visiting scientist. Permanent address: Dahan Institute of Chemical Physics, Dalian, People’s Republic of China. 2 Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. W-7405-Eng82. This article was supported by the Division of Chemical Sciences, Offme of Basic Energy Sciences.
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&H,S+ are CH3CH2Sf (I+), cis-CH&HSH+ (II+ ), trans-CH,CHSH+ (II’+ ), and CH&CH: (III+ ) (footnote 1). Based on the thermochemistry reference book compiled by Lias et al. [ 41 the heats of formation at 0 K (mfo) for these species are not known, except those for II+ (or II’+) and III+, where the AH,-,, values have been estimated from appearance energy (AE) measurements. One of the most important aspects of a photochemical experiment is to identify the structures of primary photoproducts and the branching ratios of primary product channels. When polyatomic radicals and radical ions formed in photodissociation (PD) or photoionization (PI), respectively, are de#* The calculation (ref. [ 1 ] ) indicates that CH$JHSH and CH&HSH* consist of the cis- and tram-conformers with the H attached to S cis and bans to the CH3 group, respectively.
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 213, number 3,4
CHEMICAL PHYSICS LETTERS
tected by mass spectrometry, their structures usually cannot be identified if the energetics for the poly atomic radicals or radical ions of interest are not known. In such cases, the combination of experimental measurements and reliable theoretical predictions is desirable for identifying the isomeric structures of primary photofragments. Here, we present the results of a combined experimental and theoretical study of &H,S and C2HSS+ formed in the PD and PI processes, respectively, CH3CH2SCH2CH3+hv (193 nm) -KzH$ + CzHs ,
(1)
CH3CH2SCH2CHl+h~+C2H$++C2Hg.
(2)
The CZHSSradicals produced by process ( 1) have also been studied by PI mass spectrometry according to C,H,S+ hv-C,H,S++e-
.
(3)
Quantum-chemical calculations for the CzH$Sand CLH$+ system have been made using the ab initio Gaussian-2 theoretical procedure [5-lo]. Previous investigations indicate that the G2 predictions for molecular energies, such as ionization energies (IEs ) and heats of formation (A&,,), for organosulfur molecules, neutrals and ions, are accurate to * 0.15 eV [ 7-91. After comparing the measured IE for process (3) and the AZ&, value for C,H,S+ formed in process (2) with G2 theoretical predictions, we have assigned structures for C2H,S and CzH,S+ observed in the PD and PI processes ( l)(3). When experimental IEs and A&, values for isomerit C,H,S and &H&S+ are available [ 1l-l 3 1, we find good agreement between the experimental results and the G2 theoretical predictions.
8 October 1993
seeded in Ar is produced by expansion through a pulsed valve with a nozzle diameter of 0.5 mm and a total stagnation pressure of 1150 Torr. The pulsed valve is operated at 40 Hz. The gas beam is skimmed by a conical skimmer before entering the PI region of a quadrupole mass spectrometer_ In this experiment, a supersonic beam of C2H5S radicals is prepared by 193 nm laser PD of a ( CH&H2)zS pulsed jet. The PD ArF excimer laser beam is mildly focused and intersects the ( CH,CH2).$ free jet at 90” and x 2 mm from the nozzle tip. The triggering pulse for firing the excimer laser is delayed by 500 ns with respect to the triggering pulse for opening the pulse valve. The excimer laser pulse energies used vary in the range of 35-80 mJ. Previous studies support that the internal excitations acquired by the C2HSSradicals in the PD process are relaxed efficiently as the radicals entrained in the carrier gas flow expand further into the vacuum. Photoionization sampling of the radical beam takes place xlcm from the nozzle tip. The &H&S+ ion beam pulse, observed by the PI of CzH$ formed in the PD radical supersonic source using a multichannel scaler, has a full-width-at-half-maximum (fwhm) of z 800 us [ 16,171. The C,H,S+ ion counts are measured by gating the scaler within the temporal range of 0.9-2.9 ms with respect to the triggering pulse for opening the pulse valve. The typical counting time for C,H,S+ from CzHSS at each PI wavelength is 5 min The same experimental procedures are used in the PI measurements for process (2) except that the PD excimer laser is turned off and the counting time at each wavelength is typically 15-20 s. The PI wavelength resolution used is %1.5 8, (fwhm ) . 2.2. Theoretical calculations
2. Experimental and theoretical methods 2.1. Experiment The PI study of processes (2 ) and (3) has been performed using the molecular beam PI mass speG trometer [ 14- 171. The experimental procedures used are similar to those described in the recent PI studies of CS, SO, and CH$ [ 16,173. The pulsed jet of (CH,CH,),S (vapor pressure M19OTorr at 323 K)
The ab initio G2 theoretical procedure has been described in detail by Curtiss et al. [ $61. Briefly, at the G2 level of theory, molecular structures are optimized using the MP2 perturbation calculations with the 6-31G’ basis set and with all the electrons included (MP2 (full) /6-3 1G*). AI1 single-point calculations involved are based on the MP2/6-31G* optimized structures. The G2 method, an approximation of a QCISD(T)/6-311 +G(3df, 2p) calculation, requires single-point calculations at the MP4/ 251
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6-31 lG”, MP4/6-311 +G”, MP4/6-31 lG(2df, p), QCISD(T)/6-311G”,andMP2/6-311+G(3df,2p) levels. A small semi-empirical correction is applied to account for the high level correlation effects. The MP2/6-3 1G’ harmonic vibrational frequencies, scaled by 0.93, are used for correction of zero-point vibrational energies (ZPVE) [ 91. The total energy at 0 K (Eo) is equal to E, +ZPVE, where E, is the electronic total energy. All single-point calculations have been carried out on CKAY-YMP and CRAY-2 using the GAUSSIAN 90 and GAUSSIAN 92 package programs [ 23 ] _
3. Results and discussion The PI efficiency (PIE) spectra for C2HSS+from CzHSS (process (3) ) and for CzH$+ from (CH,CH*)$ (process (2) ) are depicted in figs. la and lb, respectively. The PIE spectrum for (CH,CH&S (not shown here) has also been measured. The IE observed for (CHSCH2)zS (8.41 fO.O1 eV) is consistent with the literature value of 8.43 + 0.0 1 eV [ 41. The ionization threshold for CzHsS formed in process (1) is sharp with an IE of 8.97 + 0.0 1 eV ( 1382 + 2 A), indicating that the structure for C,H,S+ resulting from process (3) is similar to that for C2H& The previous photofragmentation studies of organosulfur molecules at 193 nm suggest that the dominant dissociation pathways involve cleavage of the C-S and S-H bonds
10
(b) 8
4
C&S+/CeHSSC,H,
r(a' */CH,CH,S CH,CH,S
Fig. I. PIE spectra: (a) CH$H$* formed in the PI of CH,CH# in the wavelength range of 1351%14OOA and (b) CIH,S+ formed intheP1 of (CH,CH,),S in the wavelengthrangeof 1150-1270A.
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[ lo-12,16,17]. Thus, the C2HSSproduced in process ( 1) most likely has the I structure and the ionization of I should yield I+. The sharp ionization onset for I is consistent with the fact that the ionization of this radical involves the removal a nonbonding electron from S [ 181. The fact that the IE for I is slightly lower than that (9.23+0.01 eV) [ 161 for CH$ can be attributed to the greater inductive effect for CH,CHZ compared to CHj. The AE for process (2) is determined to be 12232 5 A ( 10.13 20.04 eV), the energy value at the intersection of the sharp rise in PIE for CzH$+ and the background baseline, as indicated in fig. lb. The uncertainties of 5 5 A account for the finite PIES observed in the range of z 1220-1228A. Since the (C2H5)$ sample is introduced as a pulsed supersonic beam, we assume that the hot band effect is not important. Using 1224f 5 A as the AE for C,H,S+ from (CH3CH2)$, and the known values for A% [ ( CI-W% )3 1 and A&o(CI-WM 14I, the AHfovalue for C,H,S+ formed in process (2 ) is determined to be 189.6kcal/mol. Previous studies show that the mass 47 ions observed from CH,SCH, and from CHaSH have the CH,SH+ structure at their PI onsets [ 7,9,19]. Similarly, the C2HsS+ ions formed in process (2 ) are most likely II+ or II’+. In order to confirm the speculation about the assignment of I, II+ (or II”), and I+ for CzH$ and C2H5S+ formed in processes (1 ), (2), and (3), respectively, G2 calculations have been made for I, II, II’, I+, II+, and II’+. Since the energetics for III and III+ have been determined in recent experiments [ 1 1- 131, we have also performed calculations for this CzH$ isomer and its ion. Figs. 2a-2d show the structures for I and I+, II and II+, II’ and II’+, and III and III+, respectively, optimized at the MP2/63 1G* level. As expected, the structures for I and I+ are very similar and both have a C, symmetry. The ionization of I in the ground ‘A” state yields I+ in the ground 3Anstate. The removal of an nonbonding electron from S has the effect of shortening the C-S bond by 0.037 A with concomitant lengthening of the C-C bond by 0.02 A. Both the ‘A ground states for II and II’ have a C, symmetry. The ionization of II and II’ involves the ejection of the unpaired electron from the middle C atom. As a result, the bonding around the C atom is transformed from a pyramidal to a planar structure and the structures for II+ and
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II’+ in the ‘A’ ground states have a C, symmetry. Similarly, the ground state (‘A’) for III+ also results from the ionization of the unpaired electron from the C atom of the CH, group in III. Table 1 lists the G2 theoretical values for Er,, IE, and AZ&,of I, II (II’), III, I+, II+ (II’+), and III+. We find that the G2 IE (IE(G2)) value of 9.07 eV for I is in good agreement with that observed for CzHSSformed in process ( 1), supporting the expectation that the CzHSSradicals formed in the 193 nm PD of (CHsC!Hz)# have the I structure. The IE(G2) values for the other neutral isomers are less than 7 eV. The observation that the PIES for C2HSS+ are neg-
ligible below the IE for I can be taken as evidence that I is overwhelmingly the dominant photoproduct, with negligible contribution from isomers such as II (or II’) and/or III. Based on the energy release measurement of the process, CH&H$H+ hu( 193 nm)-&H,S+H, and assuming that CzHSS produced has the I structure, A&(I) is deduced to be 31.4+ 2 kcal/mol [ 171, This value, determined directly from the kinetic energy release measurement, agrees with the literature value of 33 kcal/mol determined by kinetic methods [ 2 11, These values for m,(I) are higher than the G2 value of 27.5 kcal/ mol for mro(I) by o4kcal/mol. Using Mro(I)
a
b .
&l-2-3 = -32.1
-I L6-3-2-1
105.8
=56.0 = -154.0
L4-3-2-l = 170.1
1
+
105.7 107.9
= 122.9
= 60.7
(I’) c, Fig. 2. Geometric structures: (a) CH$H$(C,; CH,CHSH+(C,; ‘A’); (c) tram-CH&HSH(C,;‘A) CH,SCH: (C,; ‘A’) (III+).
(II+)c, ‘A”) (I+); (b) cis-CH,CHSH(C,; zA) (II) and cis2A”) (I) and CH,CH$+(C.; (II’) andtrans-CH,CHSH+(C,; ‘A’) (II’+); and (d) CH$CH2(C,;‘A) (III) and
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Volume 213, number 3,4
I
L4-l-2-3
L6-3-2-l
=-177.8
I44.4 = 179.6
L4-3-2-i =164.0
(III)
c,
L43-2-1 = 122.8
(II’+) c,
(III’) c, Fig. 2. Continued.
=31.422 kcal/mol and the IE for I+I+ obtained here, we calculate a value of 238.3+2 kcal/mol for A&(1+), which is in excellent agreement with the G2 value of 236.5 kcal/mol. The cis- and trans-isomers, II+ and II’+, are predicted by the G2 theory to be more stable than I+ and III+. The AZ&Ovalues obtained by G2 calculations for II and II’ are essentially identical as are the values for II+ and II’+, Based on the comparison of G2 predictions with the experimental Mfo value of 189.6 + 1.Okcal/mol for CzHSS+ formed in process (2), we have assigned II+ (or II’+) as the structure for C&S+ formed in process (2) N2. A value xz The G2 calculation shows the AHmvalues for II’+ and II+ are essentially identical. Thus, C2HIS+ formed at the AE from (CH,CH,)$ may have the II+ and/or II’+ structures.
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for recommended 200 kcal/mol is of Lw, (CH$HSH+ ) in the most recent thermochemistry compilation [ 41. This value, based on the AE for C,H,S+ from the PI of CH$SCH3 measured by Baer and co-workers [ 201, is significantly greater than the value for A&,(11+ or II’+) determined here. In the original reference of Baer and co-workers, the structure for the observed C2H$+ from CH$SCH, is not assigned. Instead, III+ is suggested as the possible ion based on collisional activation studies and ab initio calculations [ 201. The experimental A&(111) =34.8 k2.5 kcal/mol and A&(111+) = 195.1 kcal/mol determined in a recent PI and PD study [ 121 are consistent with the value A&,JIII) = 35.3 5 1.6 kcal/mol obtained in a recent kinetic measurement [ 13 ] and the literature value A&( III+) =
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Table I Comparison of experimental values and theoretical predictions for the IE and A&c, of CH,CH,S(C,; 2A”) (I), cis-CH&HSH(Ci; 2A) (II), trans-CH$ZHSH(C,; ‘A) (II’), CHsSCH2(C1; ‘A) (III), CH,CH,S+(C,; ‘A”) (I+), cis-CH,CHSH+(C,; ‘A’) (II’), transCH,CHSH+(C,; ‘A’) (II’+), andCH,SCH: (C,; ‘A’) (III+) Species
Eo(G2) (hartree)
A&,,(GZ) ‘) (kcal/mol)
CH,CH#( C.; ‘A”) cis-CH,CHSH(Ci; ‘A) tram-CH,CHSH (C,; zA) CH,SCHI( C,; ‘A)
-476.73761 -476.72512 -476.72459 -476.72189
27.5 35.3 35.6 37.3
CH,CH2S+ (C,; 3A”) cis-CH&HSH* (C . ‘A’) trans-CH,CHSH+ ;‘C.; IA’) CH,SCH; (C tr. ‘A’)
-476.40417 -476.47441 -476.47427 -476.46987
236.5 192.6 192.6 195.3
uf0(w
)
(kcal/mol) 31.4?2
34.8L2.5 =) 35.3k1.6”’ 238.3*2 189.6f 1.0”
IE(G2) b’ (eV)
IE(exp) b, (eV)
9.07 6.82 6.81 6.85
8.91 fO.O1
6.94 =)
1
195.1 c) 200 ‘) 194s)
‘I Calculated using the Eo(G2) values for species shown in the table, the Eo(G2) values for S, C, and H given in ref. [ 91, and the A& values for S (65.6 kcal/mol), C (170.0kcal/mol), and H (51.63 kcal/mol) given in ref. [4]. b, The IE for I, II, II’, and III correspond to the formation of I+, II+,II'+, and III+, respectively. “Ref. [ll]. d)Ref. [13]. ‘1 Calculated using the A&(exp) values for III and III+. f, The experimental value may be assigned to A&( II+ ) or aH, (II’+ ) . I) Refs. [4,20], This value is recommended as AJfm(CHICHSH*) by Lias et al. (41. In the original reference (ref. [20] ), III+ is suggested as the structure for C,H,S+ formed at the AE in the dissociative photoionization of CH$SCHs (see text). s) Ref. [4].
194 kcal/mol [ 41. Since the mfo value of 200 kcal/ mol obtained by Baer and co-workers [ 20 ] is closer to AJYHfo(III+) than to A&O(II‘c or II”), it is more likely that the CzH5S+ ion observed in the PI of CH$SCH, corresponds to III+. Combining the experimental M, values for III and III+ yields a value of 6.94 eV for the IE of III.+III+, which is in good agreement with the G2 IE for III. As shown in table 1, the deviations between the G2 and experimental values for m,(I), m,(I+), A&(111), and mH,(III+ ) are less than 4 kcal/mol. In summary, the comparison between the experimental and theoretical IE and L&~ values shown in table 1 supports the conclusion that I is formed in the 193 nm PD of (CH&H2)zS and that the PI of I gives I+ at the ionization threshold. Furthermore, the C2H5S+ ions observed at the AE in the PI of (CH$Hz)$ most likely have the II+ (or II’+) structure. The utilization of high-level ab initio predictions for molecular energies is highly profitable for identifying the isomeric structures of primary polyatomic photoproducts.
Acknowledgement CYN acknowledges the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. NLM and WKL wish to thank the support of a Hong Kong University and Polytechnic Grants Committee Earmarked Grant for research (Account No. 221600080).
References [ 1] S.-W, Chiu, N.-L. Ma, W.-K. Li and C.Y. Ng, to be published. [2 ] W.J. Hehre, L. Radom, P von R. Schleyer and J.A. Pople, Ab initio molecular orbital theory (Wiley, New York, 1986). [ 3 ] M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Raghavachari, M.A. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrccs, D.J. Fox, R.A. Whiteside, R. Seeger, CF. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and JA Pople, GAUSSIAN 90 (Gaussian Inc., Pittsburgh, PA 15213, 1990).
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[4] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17, Suppl. No. 1 (1988). [ 51L.A. Curtiss, K. Raghavachari, G.W. Trucks and J.A. Pople, J. Chem. Phys. 94 (1991) 7221. [ 61 L.A. Curtiss, L.D. Kock and J.A. Pople, J. Chem. Phys. 95 (1991) 4040. [ 71 R.H. Nobes and L. Radom, Chem. Phys. Letters 189 ( 1992) 554. [R]L.A. Curtiss, R.H. Nobes, J.A. Pople and L. Radom, J. Chem. Phys. 97 (1992) 6766. [9] S.-W. Chiu, W.-K. Li, W.-B. Tzeng and C.Y. Ng, J. Chem. Phys. 97 (1992) 6557. [lo] S. Nourbakhsh, C.-L. Liao and C.Y. Ng, J. Chem. Phys. 92 (1990) 6587. [ 111S. Nourbakhsh, K. Norwood, H.-M. Yin, C.-L. Liao and C.Y. Ng, J. Chem. Phys. 95 (1991) 946,5014.
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[ 121% Nourbakhsh, H.-M. Yin, C.-L. Liao and C.Y. Ng, Chem. Phys. Letters 183 (1991) 348; 190 (1992) 469. [ 131P.H. Wine, private communication. [ 141Y. Ono, S.H. Linn, H.F. Prest and C.Y. Ng, J. Chem. Phys. 73 (1980) 2523. [ 151C.Y. Ng, Advan. Chem. Phys. 52 ( 1983) 265. [ 161 S. Nourbakhsh, K. Norwood, G.-Z. He and C.Y. Ng, J. Am. Chem. Sot. 113 ( 1991) 63 11, and references therein. [ 171 K. Norwood, S. Nourbakhsh, G.-Z. He and C.Y. Ng, Chem. Phys. Letters 184 (1991) 147. [ 181 C.-W. Hsu, C-L. Liao, Z.-X. Ma, P.J.H. Tjossem and C.Y. Ng, J. Chem. Phys. 97 (1992) 6283. [ 191 B. RuS&?and J. Berkowitz, J. Chem. Phys. 97 ( 1992) 1818. [20] J.J. Butler, T. Baer and S.A. Evans Jr., J. Am. Chem. Sot. 105 (1983) 3451. [2 I ] D.F. McMillen and D.M. Golden, Ann. Rev. Phys. Chem. 33 (1982) 493.