J . Phys. Chem. 1986, 90, 5485-5491
5485
632-22-4; DMSO, 67-68-5; TMS, 126-33-0;HMP, 680-31-9; MEOH, 67-56-1;AC, 67-64-1; PC, 108-32-7;BL, 96-48-0;TMP, 512-56-1;AN, 75-05-8; BN, 100-474 NMTP, 10441-57-3;DMTF, 758-16-7;HMTP, 3732-82-9; TDE, 111-48-8; LiC104, 7791-03-9; CU(CF,SO,)~,3494682-2; CsC104, 13454-84-7;NaCI04, 7601-89-0; KB(C6H,),, 3244-41-5; RbB(C6H5),,5971-93-7; CsB(C6H,),, 3087-82-9;T1C]O4, 15596-83-5; Zn(CF,S03)2, 54010-75-2; Cd(CF,S03)2, 29105-03-1; Pd(CF3S03!2, 27607-92-7; Ag(CF$O3), 2923-28-6; NaB(C6H5),, 143-66-8; Li+, 7439-93-2;Na', 7440-23-5;K, 7440-09-7;Rb, 7440-17-7;Cs, 7440-46-2; T1, 7440-28-0; Zn, 7440-66-6; Cd, 7440-43-9; Cu, 7440-50-8; Pb, 7439-92-1; Ag, 7440-22-4; ferrocene, 102-54-5.
dimethylthioformamide and 1-methyl-2-thiopyrrolidinone. Acknowledgment. Financial support from the Fonds zur Fordem% der wissenschaftlichen (Austria) is gratefully I* B' Kutzler for his appreciated' The author is indebted to computer software and to Mr+K*Kellner for in carrying out some of the electrochemical measurements. Registry No. FA, 75-12-7; NMF, 123-39-7; DMF, 68-12-2; DEF, 617-84-5; DMA, 127-19-5; DEA, 685-91-6; NMP, 872-50-4; TMU,
Photosensitized Reaction of Hg(3P) Atoms with the Dichlotoethenes in Krypton Matrix: Triplet Surface Chemistry Harry E. Cartland+and George C. Pimentel* Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: February 6 , 1986)
The reactions between Hg(3P) atoms with the three dichloroethenes in krypton matrix at 12 K have been studied. In the absence of Hg, matrix photolysis with wavelengths longer than 200 nm gives isomerization as well as, for cis-dichloroethene (c-DCE) and 1,l -dichloroethene (1,l-DCE) but not for trans-dichloroethene (t-DCE), HC1 elimination to give C1H-C2HCl. In the presence of Hg atoms and with excitation in the range 246-257 nm, HCl elimination is substantially reduced and, for c-DCE and 1,l-DCE, new products appear. These products are identified as Cl2.C2H,and chlorovinyl mercuric chlorides, the latter the net result of mercury insertion into a carbon-chlorine bond. The insertion product from c-DCE is identified as trans-2-chlorovinyl mercuric chloride and that from 1,l-DCE is probably 1-chlorovinyl mercuric chloride. The results indicate that in the krypton matrix, Hg(3P)-initiated chemistry takes place on a triplet surface that is not accessed with higher energy, singlet excitation. Furthermore, the absence of C12elimination or insertion chemistry for t-DCE indicates that the role of Hg(3P) is not merely energy transfer but, instead, one that opens reaction channels not observed without Hg(3P).
Introduction Excitation of group IIB (12)18 metal atoms (Hg, Cd, Zn) to the 3P state in a krypton matrix at 12 K in the presence of 2chloro-1,l-difluoroethene(CDFE) results in a single product due to metal atom insertion into the C-CI bond.' (Hereafter, ref 1 is called I). This contrasts with expectations based upon the gas-phase mercury photosensitization chemistry of vinyl fluoride and of the difluoroethenes, for which HF elimination is observedrZ and of vinyl chloride, for which HCl elimination is ~bserved.~ This difference adds interest to the matrix chemistry of the excited IIB (12) metal atoms with other halogenated ethenes. The three dichloroethenes furnish an interesting set, with different opportunities for aa and cup elimination reactions as well as the possibility of metal atom insertion into either the C-CI or C-H bonds. Although mercury photosensitization of vinyl chloride has been well s t ~ d i e dthere , ~ is litt!e such study of the dichloroethenes. Vacuum ultraviolet photolysis of the three dichloroethenes in matrices have been examined by Warren, Smith, and Guillory," who found HCl elimination to be the major reaction product. Later, McDonald et aL5 used argon resonance radiation of the various chloroethenes in argon matrices to produce HCI elimination, with specific interest in the resulting hydrogen-bonded HC1-acetylene cage complexes. However, neither the matrix photochemistry of the dichloroethylenes at wavelengths longer than 200 nm nor their Hg(3P) sensitized chemistry have been reported. We report here the krypton matrix photochemistry of the three dichloroethenes at wavelengths longer than 200 nm and their matrix reactions with the group IIB (1 2) metal atom Hg, excited to its 3P state. 'Present address: Science Research Lab, United States Military Academy, West Point, NY 10996.
0022-3654/86/2090-5485$01.50/0
Experimental Section The cryogenic apparatus, deposition technique, and infrared spectroscopy were identical with those described in I. Typically, 1 mmol of a !uypton/dichloroethene mixture at M/R = 100 was deposited on a CsI substrate at 21 K with a flow rate of 1 mmol/h. Subsequent photdlysis and spectroscopy were conducted at 12 K. Atomic Hg was added exactly as in I. Photolyses were conducted with the 1000-W Hg-Xe high-pressure arc lamp equipped as in I, either "broadband", in which case the air cut-off near 200 nm determined the short-wavelength limit, or through one of the filters described in Table I of I. The liquid dichloroethenes were obtained from Aldrich with specified purities: cis-dichloroethene, 97% (hereafter c-DCE); trans-dichloroethene, 98% (hereafter t-DCE); 1,1 -dichloroethene, 99% (hereafter 1,l-DCE). Both c-DCE and 1,l-DCE were transferred from new bottles to bulbs under nitrogen in a glovebox. t-DCE was transferred in air. All samples were degassed through 3 or 4 freeze-thaw cycles using liquid nitrogen followed by cryogenic distillation at -77 OC (solid C02/2-propanol bath), from (1) Cartland, H.E.;Pimentel, G. C. J . Phys. Chem. 1986, 90, 1822. (2) (a) Strausz, 0. P.; Norstrom, R. J.; Salahub, D.; Gasavi, R. K.; Gunning, H. E.; Csizmadia, I. G. J . Am. Chem. SOC.1970, 92(4), 6395. (b) Tsunashima, S.;Gunning, H. E.; Strausz, 0.P. J . A m . Chem. Soc. 1976,98,
1690.
(3) Bellas, M. G.; Wan, J. K. S.; Allen, W. F.; Strausz, 0. P.; Gunning, H. E. J . Phys. Chem. 1964, 68, 2170. (4) Warren, J. A,; Smith, G. R.; Guillory, W. A. J . Photochem. 1977, 7,
263.
(5) McDonald, S . A.;Johnson, G. L.; Keelan, B. W.; Andrews, L. J . Am. Chem. SOC.1980, 102(9), 2892. (6) Shimanouchi, T., Report NSRDS-NBS 39, National Bureau of Standards, US.,Gaithersburg, MD, June, 1972.
0 1986 American Chemical Society
Cartland and Pimentel
5486 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 TABLE I: Krypton Matrix Spectra of the Dichloroethenes (12 K ) Kr Y,
crn-'
trans-DCE gas6 Y, crn-'
3102.1 (rn)
909.4 (s) 823.3 (vs) 820.2 817.3
Liquid state.
Y,
3090
1199.4 (s)
a
cis-DCE
Kr
Y9
1200
YIO
900 828
Y6
cm-'
3085.9 (w) 3079.9 (m)
VI 1
Y,
crn-'
3077" 3072
Y,
1590"
u2
1299.5 (m) 1186.3 (VW)
1303 1183'
*9
851.0 (vs) 849.4 847.4 840.4 (w)
857
VI0
715.8 (m) 7 12.6 696.7 (vs) 570.4 (m)
714"
Y4
697 571
Y12
Y, v1
asym C H str sym CH str
1627
u2
C=C str
1400
Y3
1095 875 800
VI 1
ug
asym CC1 str
600.1 596.2 (m) 592.4
603
u4
sym CCl str
458.8 (m)
460
VI2
(m) (s) (vs) (vs)
ui I
* CCI, solution. I
I
I
I
I
I
K
C
VI0
C,HCl*HCI C-DCE C-DCE C-DCE
"4
C-DCE
VI2
C-DCE
"5
"2
"9
I
I I
assignt
crn-'
2778.3 (0.0008) 1591.9 (0.0024) 1302.1 (0.0020) 860.9 (0.0079) 859.2 (0.0056) 857.3 (0.0016) 718.2 (0.0024) 715.1 (0.0016) 699.6 (0.0063) O X
assignt
3130b 3035
1138.9 1087.6 867.0 785.9 784.0 782.1
u3
cm-l
Y,
3128.1 (vw) 3035.5 (vw) 1739.2 (w) 1613.8 (vs) 1555.0 (m) 1382.6 (w)
Y8
1705.5 (w) 1590.9 (s) 1565.1 (rn)
Kr cm-l
TABLE II: Products of Photolysisn of t-DCE in Kr (12 K ) Y,
1,I-DCE gas6
-
gas6
Xrll-DCE
8
10011
X>200Drn
0.00
> 200 nm.
I
I
I
I
I
I
I
3200
2800
2400
2000
1600
1200
800
which the middle fraction was used. Krypton (Airco 99.995%) was used without purification.
Y (cm-') Figure 1. Difference spectrum: 1-h unfiltered photolysis (X of a Kr/t-DCE = 100/1 sample at 12 K.
Results
TABLE III: Products of Photolysis of c-DCE in Kr (12 K)
Matrix Spectra of the Dichloroethenes. Table I lists the most prominent spectral features for each of the dichloroethenes, together with the corresponding gas-phase frequencies. Vibrational mode assignments are given where reasonably well-known. None of these samples showed any spectral changes on prolonged, filtered photolysis through an interference filter transmitting X = 246-257 nm (filter d in Table I of I). trans-Dichloroethene. One hour of unfiltered photolysis (A > 200 nm) of a 1.02 mmol matrix (Kr/t-DCE = 100) gave the difference spectrum shown in Figure 1. Table I1 lists the product band positions. Comparison to Table I shows clearly that five product features match closely both in frequency and relative intensity the five most intense features of c-DCE (in order of decreasing intensity, product bands 860.9, 699.6, 1591.9, 718.2, afid 1302.1 cm-'). The largest discrepancy occurs for 860.9 cm-', which, in the reference spectrum, is found at 851.0 cm-I. However, in this case, the product frequency is quite close to the gas-phase frequency, 857 cm-'. There can be no doubt that a major product of unfiltered matrix photolysis of t-DCE is the trans cis isomerization product, c-DCE. Figure 1 also shows some features assignable to artifacts: features due to annealing of intense parent t-DCE features (bands labeled K) and to C 0 2and H 2 0 (I and H ) and a subtraction artifact (L). A very weak absorption at 2778.3 cm-', labeled C , is the only other product band detected. A matrix with composition Kr/t-DCE/Hg = 14000/140/2 was photolyzed for 1 h through filter d (246-257 nm). The only spectral changes observed were attributable to annealing of the
-
c-DCE/Kr > 200 nm U, cm? a 3315 (0.005) 3308.5 (0.029) X
3097.8 2773.5 2776.5 2768.9 2099.9
(0.010) (0.033) (0.029) (0.005) (0.021)
1198.7 (0.026) 1197.5 (0,018) 903.8 (0.055) 902.2 823.3 (0.12) 820.2 8 17.3
c-DCE/Hg/Kr 246 < X < f57 nm Y, cm-"
assignt
3308.5 (0.003) 3267.5 (0.017) 3097.6 (0.002) 2778 (0.001)
CIH*C2HC1 C12.C2H2 t-DCE CIHC2HCI
2099.6 (0.001) 1582.7 (0.006) 1198.7 (0,010) 1197.4 (0.007) 1161.1 (0.003) 939.0 (0.006) 903.8 (0.019) 902.2 823.1 (0.038) 819.9
CIH*C2HCI
779.0 (0.006) 772.9
t-DCE
t-DCE
"I1
G
757.5 (0.004)
615.5 (0.013) 602.0 (0.014)
> 200 nm)
748.0 (0.006) 743.5 741.8 (0.005) 615.2 (0.002) 602.1 (0.002)
Absorbance given parenthetically.
D
c C
u4
t-DCE
Reaction of Hg(3P) with Dichloroethenes L
-
I
I
I
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5487
I
I
a
A L-WE
C CIH~C2HCI
C
?I
C
I
C
TABLE I V Products of Photolysis of 1,l-DCE in Kr (12 K) 1,l -DCE/Kr 1,l -DCE/Hg/Kr X > 200'nm 246 C X 2?7 nm Y , cm-' a Y , cm-' a assignt 3308.5 (0.078)
3308.5 (0.0061 3267.7 (o.oo6j 3079.7 (0.001) 2774.3 (0.005) 2099.6 (0.003) 1596.0 (0.004) 1590.5 (0.005)
n
3079.5 (0.008) 2778.5 (0.081) 2099.9 (0.046)
0.00
n i c - n c e s io011 I
3200
F I
I
2800 2400 2000 1600 1200 Y(cm-') I
- D C12'C2H2 - D t-(CHCICHlHgCl
1590.5 (0.018) 1565.1 (0.003) 1299.5 (0.005) 1197.6 (0.009)
I
I
I
I
I
800
I
I
903.8 (0.012)
b
851.0 (0.048) 849.4 (0.031) 847.4 (0.007) 823.3 (0.020) 820.2 (0.012) 817.3 (0.002) 757.3 (0.012) 749.8 (0.003)
G
-
0.03
0.00
I
I
1299.0 (0.002) 1198.4 (0.004) 938.9 (0.002) 903.8 (0.008) 896.1 (0.006) 850.8 (0.015) 848.9 (0.011) 847.2 (0.002) 822.8 (0.014) 819.9 (0.010) 817.0 (0.003)
Ef/C-DCE/HE S 14000114011 2 4 6 < X < 257nm I I I I
747.6 (0.006) 741.8 (0.004) 730.5 (0.002) 715.6 (0.004) 712.4 (0.003) 699.8 (0.006) 696.7 (0.015) 615.4 (0.002) 602.0 (0.004)
parent t-DCE. Subsequent unfiltered photolysis (X > 200 nm) produced the same product absorptions seen in the absence of Hg. Evidently, excitation of Hg in krypton matrix containing t-DCE causes no photochemistry. cis-Dichloroethene. Three and one quarter hours of unfiltered photolysis (A > 200 nm) of a 1.0 mmol matrix (Kr/c-DCE = 100) gave the difference spectrum shown in Figure 2a. Table I11 lists the product band positions. Again, comparison to Table I shows that four product features match closely both in frequency and relative intensity to t-DCE absorptions (823.3, 903.8, 1198.7, and 3097.8 cm-'). Barely detectable (and not visible in Figure 2a) were weak absorptions at 867.0 and 785.7 cm-I, close to the two strongest bands of 1,l-DCE. Just as with t-DCE, unfiltered matrix photolysis of c-DCE causes isomerization to form t-DCE. In addition, there is a small amount of isomerization to form 1,l -DCE. In contrast to t-DCE, unfiltered photolysis of c-DCE gives another major product, C in Figure 2a, characterized by absorptions at 2778.5, 3308.5, 2099.9, 602.0, 615.5, and 757.5 cm-l (decreasing intensity). Figure 2b shows the result of 3 h of filtered photolysis (filter d, 246-257 nm) of a matrix sample (Kr/c-DCE/Hg = 14000/140/1). The product frequencies are listed in the second column of Table 111. The most prominent product is again t-DCE (absorptions at 823.1, 903.8, 1198.7, and 3097.6 cm-I). The absorptions of the compound C obtained with unfiltered photolysis are present but barely detectable. This loss of product C is accompanied by six new features that we associate with two new products labeled D (3267.5 and 748.0/741.8 cm-I) and G (1582.7, 939.0, 772.9 and 1161.1 cm-I). Thus we find that excitation of Hg in krypton matrix containing c-DCE essentially eliminates the major product C obtained from unfiltered photolysis and adds two new products D and G . I,l-Dichloroethylene. Three hours of unfiltered photolysis (X > 200 nm) of a 1.0 mmol matrix (Kr/l,l-DCE = 100) gave the difference spectrum shown in Figure 3a. Table IV lists the product band positions. Immediately evident are intense product absorptions due to the isomerization product c-DCE (851.1,696.7, 1590.5, 715.8, 3079.9, 1299.5, and 1565.1 cm-I). Weaker bands show that t-DCE is formed in a smaller amount (823.3, 903.8,
a
(0.014) (0.009) (0.024) (0.044) (0.032) (0.046)
ClH.C-,HCl CI2.C2H2 C-DCE ClHC2HCI CIH.C,HCl
Y, Y; ~g
C
C
~2
G'
YIO
C-DCE C-DCE C-DCE t-DCE
L J ~
t-DCE
~ 1 0
C-DCE C-DCE C-DCE t-DCE t-DCE t-DCE ClH*C2HCl
~2 ~g
G'
G'
VI1
C
I
715.8 712.6 699.9 696.7 615.7 602.0
C D
LJ~
D G'
u5ip V50P
CI2.C2H2
~q
C-DCE C-DCE C-DCE C-DCE ClHC2HCl ClHC2HCl
~ 1 2
C
~q
Absorbances given parenthetically. L
,
I
I
I
:i i " 0.001
I
1
I
I
C UHC2HU
I
I
I
3200 2800 2400 2000 1600 I200 Y (cm-l) L
,
I
I
I
b
200 nm in krypton gives the same product, C1HC2HCl, obtained through vacuum UV photolysis of these same compounds in argon. Zdentity o f D . The absorption frequencies of D, 3267.5, 748.0, and 741.8 cm-I, are close to those of acetylene isolated in krypton, 3280.4 and 732.5 cm-l, and point to its identity, C1,C2H2. The frequency differences are attributable to shifts due to the C1, cage neighbor. We conclude that matrix excitation of Hg atoms in the presence of cis- or I ,I-CDE causes Cl, elimination to form a matrix complex Cl2.C,H2. Another possible mechanism would be the successive abstraction of two chlorine atoms to form, first, HgCl and then HgCI,. It is conceivable that the observed C2H2frequency perturbations are due to HgC1, as a nearest neighbor. The highest vibrational frequency of HgC1, in Kr matrixlo falls at 41 1.5 cm-I and was not detected, but this frequency is near the end of our sensitivity range. The reaction of Hg(3P) with cis-DCE to form HgCl and C2H,Cl would be about 47 kcal/mol exothermic, but we expect that there would be a modest activation energy and, hence, we disfavor this mechanism. The frequency shifts and splittings provide clues to the structure of D, the C1yC2H2cage pair. The C-H stretching absorption appears 12.9 cm-' below the 3280.4-cm-' v3 frequency of C2H2 isolated in Kr.9 This is considerably larger than the 5.3-cm-' shift reported by Andrews et aL8 for the C-H stretching absorption of HC1.C2H2 in Ar. Turning to the v5 C-H bending mode of acetylene, the CIH.C2H2complex breaks the symmetry to produce a doublet with splitting 14.4 cm-' and an average shift of +7.5 cm-' from u5 of C2H2isolated in argon,8 736.9 cm-'. Product D displays for this band a smaller splitting, 6.2 cm-I, but a larger average shift, +12.4 cm-', from v5 in Kr9, 732.5 cm-]. There are two plausible, T-shaped structures of Cl,-C2H2 CI
I
CI
H-&C-H Di
D2
Structure Dl, which resembles that deduced by MacDonald et for C1HC2H2,is also in accord with the similar geometry deduced by Fredin and Nelander" for the CI2C2H, complex. Structure D2, suggested by the acidity of acetylenic C-H bonds, provides an explanation for the larger downward shift of the C-H
(7) Berry. M. J. J . Chem. Phys. 1974, 61, 3114. (8) Andrews. L.; Johnson, G. L.; Kelsall, B. J J . Phys. Chem. 1982,86,
3374. (9) Cartland, H. E., Ph D. Dissertation, University of California, Berkeley. CA, 1985.
(IO) Loewenschus, H.; Ron,A,; Schnepp, 0.J. Chem. Phys. 1969, 50, 2502. (1 I ) Fredin, L.; Nelander, 8 . J , Mol. Struct. 1973. 16, 205.
Reaction of Hg('P) with Dichloroethenes
The Journal of Physical Chemistry. Vol. 90.No. 21. 1986 5489
TABLE V
Comrrison of the Infrared Spectra (v. coil) of M u & G and Prototype Chlorovinyl Mercuric Chlorides (CHCI==€H)HgCI'
cis
trans
3032 im) I590 (m) 1268 (m) I127 (m) 909 (m)
3028 i w ) 1573 & j 1276 (w) 1156 (s) 936 (s)
769 (s)
0
G'
1582.7 (0.006)
1590.0 (0.004)
1161.1 (0.003) 939.0 (0.006)
100
772.9 (0.006)
772 (s)
'As listed in ref
1 L
730.5 (0.002) 695 (s) 592 (w) 330 (s)
.......
-I
tci?mcn=
938.9 (0.002) 896.1 (0.006)
J[CI2]'. ECiCB
668 (m)
5
330 (6)
IZb.
stretching mode and, perhaps the larger average shift of us. In conclusion, the sharp absorptions of D indicate a well-defined structure whose frequencies identijy D to be Cl,:C,H,., The different pattern in u3 and vS shifts and splittings might point to structure D, but not decisively. We conclude that ClzCzHzis a T-shaped molecule with structure D, somewhat more likely rhan D,. Idenriry of G. Since two of the three possible elimination products have k e n observed, it is reasonable to consider the possibility that G (or G') might be the third, H&Cl,. However, noneof the G absorptions, 1582.7, 1161.1,939.0, or 772.9 cm-I, matches the 995.992, and 989 c n - l absorption of HCIC,CI, as recorded by MacDonald et al.' (which we have ~ o n f i r m e d ) . ~ The remaining alternative is Hg atom insertion analogous to that observed in I. Fortunately, Nesmeyanov et al." recorded the infrared spectra of both cis- and trans-(CHCI=CH)HgCI. Table V compares these spectra to that of G. The four absorptions of G are quite close to the four strongest features of tram(CHCI==CH)HgCI (average frequency difference, 4.7 cn-'). In contrast, two of the G frequencies, 1161 .I and 939.0 cm-I, differ significantly from the corresponding frequencies of the cis(CHCI=CH)HgCI (respectively by 34 and 30 cm-'). We conclude that matrix excitation of H g in the presence of cisCHCI-CHCI gives trans-(CHCI=CH)HgC/. Identity of G'. Again the most likely identification of G' is a mercury atom insertion into one of C-CI bonds. Table V shows that G' is neither cis- nor frans-(CHCI=CH)HgCl, either of which would require isomerization. We conclude, by analogy, that Hg atom insertion into a C-CI bond has occurred and G' is (CHz==CCI)HgCI. Direct Photolysis of DCE. The heats of formation of the three DCEsdiffer very little: trans-DCE, +1.2; cis-DCE, +I.@ 1,lDCE, +0.6 kcal/mol.13 There are three possible diatomic elimination products whose energies relative to cis-DCE can be estimated from known heats of formation" or from standard bond Thus, we place HCIC,HCI at +23.5 kcal/mol (including a -1.8 kcal/mol hydmgm bond interaction" between HCI and C,HCI). Similarly, Hz-CzHCI) is a t +39 and Cl,-C,H, is at +51 kcal/mol (the latter including an assumed -1 kcal/mol interaction). Figure 4 shows the energy relationships for cis-DCE (except for isomerization). Berry' has collected the available information about the transitions that may contribute to the absorption spectra of the three DCEs in the spectral range 14W220 nm. For broadband photolysis with A > 155, he decides that "the lowest singlet n* n excitation is primarily responsible for reactant state preparation pertinent to HCI molecular photocliminations." He also calls attention to the asymmetrical spectral contours on the long-
-
(12)(a) Ncsrncyanov, A. N.; Bo-. A. E.; Ndkova. N. V.: Fdin. E. I. 1. Omommer. Chem. 1968.15.297, ib) Ncsmmnov. A. N.: AlclrPanvan. V. T.;Lite. L. A,; Prokof&, K., R&umova,'E. R.'; Bori-, A. E.'k Akod. Nouk. SSSR Srr. Khim. 1980. IO. 2297. (13) Joshi. R. M.J. Mocromol. Sei. Chem. 1974,18(5), 861. (14)Barns, A. J. J. Mol. Srruer. 1983,100. 259.
A.
-
d.-CEC
Figure 4. Energy diagram for unfiltered photolysis of cis-DCE in Kr in the absence of Hg.
wavelength sides (A > 200 nm) of the DCE absorptions that peak near 192 nm and he proposes that other transitions are active there (e.& u* n, n* n). It is to bc noted that our excitation is restricted to this long-wavelength region in which excitation other than n* Il may he significant. Figure 4 also shows the singlet and triplet states energetically accessible to cis-DCE when excited with photons of energy betwcen that of the 0 wavelength near 230 nm (124 kcal/mol) and the air cut-off at about 200 nm (143 kcal/mol). All three elimination products are energetically accessible as well as the triplet states HCI.'[C,HCI]*, 3[CI,]*C,H,, and '[CHCICHCI]* (as estimated in I). We assume that vibrational deactivation in the I[CHClCHCl]* manifold will be rapid compared to singlet-triplet intersystem massing. If so, entry to the product triplet states would be at the 0 energy of l[CHCICHCI]*, 124 kcal/mol. Since this is below the HCI.3[C,HCI]' 0 4 energy, HCI elimination could not involve this triplet state. However, the '[CIz]'~CzHz state is accessible. Thus, triplet surface elimination would favor CI, and not HCI elimination. Since the opposite result is obtained experimentally, we conclude that triplet states are not involved. Again, both the HCI and CI2elimination manifolds have excited singlet states within reach, and H, elimination undoubtedly does not. The first excited singlet state of HCI, the A ' n continuum, begins absorbing at 44000 cm-l, hence requiring photons of A < 194 nm, somewhat beyond the air cut-off. However, literature spectra" indicate that the '[GHCl]* energy is about 115 kcal/mol above the ground state, so HCI.l[C,HCI]* lies close to the air cut-off of our photolysis source. Chlorine has continuous absorption due to a ,& stateI6 that displays an indistinct onset and maximum absorption at 30500 cm-l. Adding this energy to the CI,.C,H, energy places this state below 138 kcal/mol. Thus, on the excited singlet surface reached through '[CHCICHCI]* excitation, either HCI or CIz elimination is p i b l e . However, either reaction would have to occur with an activation energy near or
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(IS) Evans. K.: Schcm. R. S.:Rice. S. A,: Hellcr. D.J. Chem. Sm.. Foridby Tlonr. 2 1973,69(6), 856. (16)Huber, K.P.;Herzberg, G. Cornrants of Diaromle Malcculer; Van Nostrand Reinhold New York, 1979.
Cartland and Pimentel
5490 The Journal of Physical Chemistry, Vol. 90. No. 21, 1986
+ a)
L L
-'-
Figure 5. Energy diagram for filtered photolysis of cis-DCE in Kr with Hg present. below 20 kcal/mol, much below the 72 kcal/mol barrier to HCI elimination found for vinyl chloride on the ground-state surface. Another possible mechanism is initial crossover from '[CHCICHCI]* into high vibrational excitation of the ground singlet state of cis-DCE. That would point to chemistry on the ground-state singlet surface with ample energy to open all three elimination channels, despite activation energies. This reaction channel is surely open under I' II excitation at 192 nm, as shown by high HCI vibrational excitation found through chemical laser emission.7J' None of these mechanisms provides a facile explanation of the absence of CI, elimination from any of the DCFs and the presence of HCI elimination from cis-DCE and 1.1-DCE under direct photolysis. Hg.DCE Photochemistry. The most striking aspect of the H g D C E photochemistry is the change of products. The elimination product CIH.C,HCI obtained from singlet DCE excitation is lost, and a new elimination product D, C12C2Hz,appears along with the mercury insertion product G . (HCICCH)HgCI, or G', (H,CCCI)HgCI. The elimination product change may be attributed to the different photon energy, the different reaction surface, or both. Figure 5 shows the energetic situation, with the various energies estimated as discussed above and, in somewhat greater detail, in I. With the Hg('P) energy, 112 kcal/mol, the only elimination channel accessible on the triplet hypersurface can lead to the observed product D,ClzC,H,. In addition, the Hg('P) insertion chemistry is possible and, just as observed in I, insertion into the C-CI bond takes place but not into the C-H bond. Of particular interest is the absence of Hg('P) reaction with trans-DCE expect for isomerization. Yet, we expea that cis- and trans-DCE have one and the same, nonplanar triplet state. We are led to the conclusion that '[CHCICHCI]' provides a plausible explanation for the cis s trans isomerization but that simple energy transfer from Hg('P).[CHCICHCI] to Hg(lS).'[CHCICHCI] * is nor involved in rhe CI, elimination or the inserrion reaction channels.
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(17) Berry, M.J.: Pimcntcl, 0. C. J. Chcm. Phys. 1910, 53. 3453. (18) In this pper the periodic group notstion in prcnthis in accord with r m n t actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated bxauw of wide confusion. Groups I A and IIA bccnme group 1 and 2. The d-transition elementscomprise group 3 through 12, and the pblack elements comprise group 13 through 18. (Note that the former Roman number designation is preserved in the Inst digit of the new numbering: e.&. 111 3 and 13.)
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m
[CHCICH]H~CI
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Jm
[ C H ~cci] nect
Fimre 6. Branching ratios and energy storage in the matrix photolysis of the drchloroethenco H i t h and uithout mercury present.
The products from 1.1-DCE add another facet. Both isomerization products cis- and trans-DCE are detected as minor products, along with the two elimination products C, CIHC,HCI, and D, CI,C,H,. and as well. the insertion product G'. This product array suggests that in the case of I.I-DCF: C-CI bond rupture within the cage IS involved (see Figure 4). followed by CI abstraction (to give CI,C,H,). H abstraction (to give CIH. C,HCI). or isomerization via recombination. If this is B correct inference, however. it is not transferable to cis. and trans-DCE. since it would again lead to identical products from the two parents. Finally, we have the observation that cis-DCF. gives the trans Hg insertion product, whereas no insertion occurs for tram-DCE. Proximity of the two halogen atoms may be needed to facilitate reaction with Hg('P), as would be required if the mechanism lint involved electron donation by the two chlorine atoms into the two vacant 6p orbitals of the Hg('P) atom. Then. as insertion procgds on the triplet surface. a nonplanar conformation permits steric and/or electrostatic factors to operate during relaxation. Branching Rarias and Energy Srorage. Figure 6a shows the experimental branching ratios for direct (singlet surface) photcchemistry at A > 200 nm. The product HCIC,HCI, which stores only about I590of the photon energy, is obtained in 8'% yield (relative) from cis-DCE and 69% yield from 1.1-DCE. Figure 6b displays quite a different picture for the triplet surface photochemistry at A 250 nm. The main product, CI,C,H,, stores almost 50% of the photon energy. and it is obtained in substantial yield From both cis-DCE and 1.1-DCE along with their exothermic insertion products. Comparison 10 Earlier Work. There are interesting differences between t h s e data and the matrix results obtained with photolysis of the dichloroethylenes at shorter wavelength and in the absence of mercury atoms. Both Warren et al.' and McDonald et al.' found CIHC,HCI to be the major product at A < 200 nm. which we found with singlet excitation at A 3 200 nm. Both groups reported. however, that the same products were obtained with rrans-DCE as well as minor amounts of CI, and H, elimination products. Warren et al.' state that 'each isomer produces the same products with the same relative yields". This outcome and the results of McDonald et al..' which show differences only in relative yield for A < 200 nm. contrast sharply with our obscrvations that show obvious differenas in products among the thrcc isomers with longer wavelength photolysis. I t may be that this difrcrcncc indicates that the electronic excitation that accounts
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J . Phys. Chem. 1986, 90, 5491-5494
for the peak absorption at X 192 nm (presumably II 11) is not the dominant excitation of X > 200 nm. Warren et alS4examined relative yields in matrices of Ar, Nz, CO, and Kr. They interpreted their observed differences in photolysis efficiencies to be evidence that "all processes, other than formation of CI2and A (acetylene) occur through triplet states". Plainly the present work furnishes more direct evidence about the role of triplet reaction surfaces, evidence that points to elimination chemistry that is determined by the singlet or triplet character of the initial excitation. Finally, the gas-phase mercury photosensitizations of the difluoroethenes present an interesting comparison to our matrix results for the dichloroethenes. Cis-trans isomerization is observed, the only similarity. Molecular HF elimination does occur2 and there has been no evidence for Fz elimination, in contrast to our matrix results for cis-CHClCHCI, where C12 elimination is the dominant channel. +
Conclusions The most striking outcome of this study is the change of products going from direct photolysis of the dichloroethenes to Hg(3P) excitation. This observation indicates that the Hg(3P)-initiatedchemistry takes place (at least in the Kr matrix) on a triplet surface that is not accessed with the higher energy, singlet excitation.
5491
A corollary question that follows is whether the surface so accessed involves Hg(3P) atoms in a participatory role beyond those of defining the initial spin multiplicity and providing, through energy transfer, the required energy. There are two pieces of evidence that point to a participatory role. First, the formation of mercury insertion products G and G' shows that Hg(3P) certainly has some chemical role in the matrix. The second evidence is connected with isomerization, which is the expected outcome of the simple energy transfer Hg(3P).(CHCICHC1) Hg(S). 3[CHClCHCI]*. If this energy transfer is the only role of Hg(3P), then cis- and trans-CDEs should have identical elimination chemistry. Since cis-DCE forms CI2.C2HCl and trans-DCE does not, we deduce that CI2.C2HCI is not a product of simple energy transfer. We conclude that Hg(3P)interacts with cis-CHCICHC1 to define chemical reaction channels that do not exist in absence of Hg(3P),one channel leading to Hg insertion into the C-C1 bond and the other leading to CIz elimination.
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Acknowledgment. This work was supported by the Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy, under Contract NO. DE-AC03-76SF0098. Registry No. t-DCE, 156-60-5; c-DCE, 156-59-2; 1,l-DCE, 75-35-4; C2HC1, 593-63-5; C2H2, 74-86-2; (CH,=CCl)HgCI, 61906-99-8; trans-(CHCl=CH)HgCl, 1190-78-9; CI2, 7782-50-5; HCI, 7647-01-0; Hg, 7439-97-6.
Polarizability and Second Hyperpoiarizabillty of Some Quinones. Analysis of the Effect of Intramolecular Interactions on the Hyperpolarizabiiity M. G. Papadopoulos* and J. Waite National Hellenic Research Foundation, 48, Vas. Constantinou Avenue, GR-116 35 Athens, Greece (Received: March 1 1 , 1986; In Final Form: April 30, 1986)
The polarizability,a,and second hyperpolarizability, y, of p-benzoquinone, 1,2- and 1,Cnaphthaquinones, acenaphthaquinone, phenanthraquinone, and anthraquinone have been computed by employing the CHF-PT-EB-CNDO method. The results are analyzed by employing a simple physical concept.
Introduction Molecular electric polarization is of current intensive interest.'-' Among the properties which measure the nonlinear response of a molecule to an electric field is the second hyperpolarizability y. This property, in addition to its significance as a fundamental molecular constant, is of particular importance as it can provide new information concerning the electronic structure of molecules, the intermolecular forces, electronic interactions within molecules, chemical reactivity, and the many applications of nonlinear optical The hyperpolarizability has also been used as a probe to investigate the stability of both neutral and charged molec u l e ~ . ~However, J~ the accurate theoretical determination of y
for molecules is a task of considerable difficulty, since one needs to satisfactorily solve problems related to the selection of appropriate basis sets, the inclusion of the important correlation effects, etc.l'J2 It is thus a worthy goal to develop and carefully check computational procedures for the reasonably accurate and economic (in terms of computer time and storage) calculation of y for molecules of chemical interest. Bearing this in mind we have made several proposals which facilitate the reliable determination of static electric hyperpoIarizabilitie~.'~-'~These proposals constitute the computational framework (CHF-PT-EB-CNDO) which here
(1) Shelton, D. P.; Mizrahi, V. Chem. Phys. Lett. 1985, 120, 318. (2) Duddley, J. W., 11; Ward, J. F. J. Chem. Phys. 1985, 82, 4673. (3) Waite, J.; Papadopoulos, M. G. J . Chem. Phys. 1985, 83, 4047. (4) Bishop, D. M.; Lam, B. Chem. Phys. Lett. 1985, 69, 120. (5) Liu, S.; Dykstra, C. E. Chem. Phys. Lett. 1985, 119, 407. (6) Diercksen, G. H. F.; Sadlej, A. J. Chem. Phys. Lett. 1985, 114, 187. (7) Pantinakis, A,; Dean, K. J.; Buckingham, A. D. Chem. Phys. Leff. 1985, 120, 135. (8) Buckingham, A. D.; Orr, B. J. Q. Rev. Chem. SOC. 1967, 21, 195. (9) Waite, J.; Papadopoulos, M.G. J . Mol. Struct. 1984, 125, 155.
1985, 81, 433.
(10) Waite, J.; Papadopoulos, M. G. J . Org. Chem. 1984, 49, 3837. (11) Waite, J.; Papadopoulos, M. G. J . Chem. SOC.,Faraday Trans. 2
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(12) Bartlett, R. J.; Purvis, G. D., 111 Phys. Rev. 1979, 20, 1313. Phys. Scr. 1980, 21, 255. Purvis, G. D., 111; Bartlett, R. J. Phys. Reu. 1981, 23, 1594. (1 3) Nicolaides, C. A.; Papadopoulos, M. G.; Waite, J. Theor. Chim. Acta 1982, 61, 427. (14) Papadopoulos, M. G.; Waite, J.; Nicolaides, C. A. J . Chem. Phys. 1982, 77, 2527. (15) Waite, J.; Papadopoulos, M. G.; Nicolaides, C. A. J . Chem. Phys. 1982, 77, 2536.
0 1986 American Chemical Society
