Photochemistry of Deuterated Acetylketenes: Matrix Isolation Infrared

Photochemistry of Deuterated Acetylketenes: Matrix Isolation Infrared ... Irradiation of Ar matrices in the UV (313 nm) caused the appearance of new s...
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J. Phys. Chem. 1996, 100, 3917-3922

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Photochemistry of Deuterated Acetylketenes: Matrix Isolation Infrared Spectroscopic and ab Initio Studies Ralf H. Zuhse, Ming Wah Wong,* and Curt Wentrup* Department of Chemistry, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed: September 22, 1995; In Final Form: NoVember 27, 1995X

The partially deuterated s-Z and s-E conformers of acetylketene (1 and 2) were generated by flash vacuum pyrolysis of two different deuterated precursors, 3 and 4. The conformer mixture was characterized by lowtemperature FTIR spectroscopy in Ar, N2, and Xe matrices. The observed frequencies and intensities are in excellent agreement with ab initio calculations at the MP2/6-31G* and B-LYP/6-31G* levels. Irradiation of Ar matrices in the UV (313 nm) caused the appearance of new sets of absorptions, which disappeared upon annealing of the matrices to 35 K. Difference spectra revealed no new absorptions due to OD bands arising from photoisomers 7-10, thereby rigorously proving that no C-D/O-D tautomerism takes place and that the new absorptions observed are due to exceptionally prominent photochemically populated matrix sites.

Introduction

SCHEME 1

The R-oxo-ketenes are highly reactive intermediates and their widespread potential in organic synthesis1 has been the rationale for mechanistic,2,3 spectroscopic,2a-d,3a,4 and computational studies5 in recent years. One of the most versatile current methods of investigation of reactive compounds of this type is matrix isolation IR spectroscopy at cryogenic temperatures.2a,b,4a-f,i,7 The advantage of this technique lies in the stability of the intermediates in the matrix and the absence of rotational absorptions, allowing not only the detection of a particular species in a mixture but also the possibility of distinguishing between individual conformers. Previous work on the matrix isolation of acetylketene2a,3a,4a,6 allowed the assignment of the IR spectra of the individual s-Z and s-E conformers. However, it was also discovered that the photolysis of Ar matrix isolated acetylketene at ca. 14 K resulted not only in s-E f s-Z conversion but also in the development of new IR bands in the ketene region, which disappeared again on annealing to 35 K.4a,6 These bands were ascribed to photoinduced sites. In contrast, it has been reported that similar Ar matrix photolysis of formylketene (HCOsCHdCdO) resulted in new bands (3526, 2128, 1692 cm-1), ascribed to the hydroxymethyleneketene tautomer (HOsC(H)dCdC)O), formed in a photochemical 1,3-H shift.7 The medium-strength OH absorption at 3526 cm-1 is particularly noteworthy. It could be argued that acetylketene should exhibit analogous photochemical behavior and that the OH band in the phototautomer (cf. Scheme 2) had been obscured by unavoidable traces of water. Morever, although the computed classical barriers for tautomerization of the enols back to the R-oxoketenes (Scheme 2) are very substantial (126-142 kJ mol-1; see below), it is conceivable that these tautomerizations could occur by tunneling at 35 K if the barriers are sufficiently narrow. Deuteration should result in a large isotope effect on this rate. The present study was undertaken in order to establish conclusively the absence or presence of a 1,3-hydrogen shift and hence formation of enol tautomers (cf. Scheme 2) during the photolysis. We report the flash vacuum pyrolysis (FVP) and matrix photolysis of two partially deuterated precursors 3 and 4. By using deuterated acetylketenes, a 1,3-shift would X

Abstract published in AdVance ACS Abstracts, February 1, 1996.

0022-3654/96/20100-3917$12.00/0

lead to an OD absorption which would be easily detectable and distinguishable from H2O. The OD absorption would be expected in a region free of other interfering absorptions (ca. 2500-2600 cm-1, see Table 3). If hydrogen tunneling was the process observed on annealing to 35 K, a large deuterium isotope effect should be seen. To aid the interpretations of the IR spectra, ab initio calculations were performed at the MP2/631G* and B-LYP/6-31G* levels of theory. Results FVP of 3 and 4 (Scheme 1) was studied in the temperature range 450-650 °C with isolation of the products in an argon matrix at 15 K and monitoring by FTIR spectroscopy. As in the case of the protio compounds, the precursors 3 and 4 pyrolyze to give the deuterated ketenes 1 and 2 in a s-Z/s-E ratio of roughly 1:1 (based on the IR intensities). There is no noticeable dependence of this ratio on the pyrolysis temperature. Also, apart from the major ketene absorptions, all three © 1996 American Chemical Society

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Zuhse et al.

TABLE 1: Infrared Absorptions and Intensities of the Deuterated Acetylketenesa,b in the Ketene Region during a Photolysis-Annealing Cycle CH3sCOsCHdCdO (11)

CH3sCOsCDdCdO (1)

CD3sCOsCHdCdO (2) 2144 2148 2135 2140 2136e 2132f 2125g

2143 2148 2133 2137

(100) (43) (81) (15)

2137 2144 2127 2131

2130 2133 2137 2143 2148 2149 2154

(54) (71) (17) (100) (34) (71) (47)

2123 2127 2131 2137 2144 2147 2151 2154

2143 2148 2133 2137

(100) (70) (83) (65)

2137 2144 2127 2131

(100) (61) (78) (41)

UV Irradiation with 313 nm for 2.5 h (50) 2120 (38) 2125 (24) 2132 f (69) 2135 (100) 2136 e (29) 2144 (53) 2148 (18) 2151 Annealing to 35 K for 2 min (100) (89) (73) (78)

2144 2148 2135 h 2140

(93) (14) (100) (27) (95)

s-Z s-E

majorc minord major minor

(13) (17) (59) (53) (61) (100) (35) (82) (100) (80) (83) (88)

s-Z s-E

major minor major minor

a Frequencies in cm-1. b Intensity values (in % relative to the strongest absorption) are in parentheses. c Strongest absorption (major site) in the ketene region pertaining to an individual conformer. d Weaker absorption (minor site). After irradiation these “minor” bands can become stronger than the “major” bands. e C-D symmetrical stretching band of CD3sCOsCHdCdO (see Table 7). f Absorption of s-E-7. g Absorption of s-Z-7. h Broad absorption due to 2136 and 2135 cm-1 components (see Table 7).

TABLE 2: Comparison of Experimental and Calculateda Ketene Absorption Bands (cm-1) of Deuterated Acetylketenes s-Z conformer CH3sCOsCHdCdO (11) CH3sCOsCDdCdO (1) CD3sCOsCHdCdO (2) CD3sCOsCDdCdO (6) a

TABLE 3: Calculated OH and OD Stretching Frequenciesa (cm-1) of Acetylketene Tautomers

s-E conformer

calc

exp

calc

exp

2145 2137 2146 2137

2143 2137 2144 2132

2134 2126 2134 2126

2133 2127 2135 2125

MP2/6-31G* values; scaled by 0.960 (ref 6).

acetylketenes (Table 1, including the protio compound 11) showed minor bands at higher wavenumbers next to the main absorptions. These minor absorptions are always present, regardless of the structure of the precursor and the pyrolysis temperature. The intensities of these minor bands undergo considerable change dependent on the argon/solute (M/S) ratio and deposition rate and are evidently due to a matrix site effect. In earlier work from this laboratory using difference spectra before and after photolytic interconversion of the conformers in the argon matrix, all absorptions of the individual conformers were assigned.4a,6 In addition, ab initio calculations were carried out to aid this process.6 These data, together with our own ab initio calculations, reported herein, made it possible to assign most of the IR bands with significant intensities in the range of our experiments (4000-750 cm-1) for both conformers. We have previously used ab initio calculated vibrational frequencies for comparison with experimental values. Based on comparison between MP2/6-31G* and experimental frequencies for several molecules containing the CdCdO functional group,6 a scaling factor of 0.960 instead of the standard value of 0.94279 is found to be more suitable for the ketene stretching vibration at the MP2/6-31G* level. The results for acetylketenes are given in Table 2. Density functional theory (DFT) is developing rapidly as a cost-effective method for studying ground state molecular properties.10 Here we have employed the B-LYP formulation of density functional theory to examine the IR spectra of the acetylketenes. As demonstrated in Table 3, the B-LYP/6-31G* frequencies are in good agreement with

species

protio compound D-isotopomer D3-isotopomer

hydroxymethyleneketene 3465 (12) s-E hydroxyvinylketene 3540 (13) s-Z hydroxyvinylketene 3523 (13) a

2522 (7) 3540 (8) 3523 (8)

3465 (9) 2576 (10) 2564 (10)

MP2/6-31G* values, scaled by 0.9427 (ref 9).

TABLE 4: Ketene and Carbonyl Frequencies (cm-1) of Deuterated s-Z- and s-E- Acetylketenes in Different Matrix Enviroments νCdCdO

νCdO

s-Z-1

νCdCdO

νCdO

s-E-1

N2

2141 m 2143 s

1682

Xe

2133 s 2134 m 2140 m 2146 w

1669

s-Z-2 / s-Z-6

2127 m 2130 s 2135 m 2121 m 2125 m

1693 1677

s-E-2 / s-E-6

N2

2142 s 2148 s

1686 1688

Xe

2130 s 2132 m 2138 s

1676 1683

2124 w 2128 m 2130 m 2138 s 2139 s 2121 w 2128 m 2133 w

1679 1682

1686 1689

experimental data from the Ar-matrix isolated species. The rms deviation between the B-LYP/6-31G* and observed frequencies is 18 cm-1. The predicted IR intensities are also in good qualitative agreement with the observed spectra. Note that the B-LYP theory provides a better prediction of the ketene stretching vibrations but a somewhat poorer description of the C-H stretching vibrations of the acetylketenes. To assist the interpretation of the experimental spectra, infrared spectra were calculated for CH3sCOsCDdCdO (1) and

Photochemistry of Deuterated Acetylketenes

J. Phys. Chem., Vol. 100, No. 10, 1996 3919

TABLE 5: Calculated and Experimental Infrared Spectra of the Undeuterated Acetylketenesa s-Z-11 ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13

s-E-11

MP2/6-31G* b

BLYP/6-31G*c

3098 (12) 3048 (5) 3008 (6) 2929 (2) 2107 (604) 1652 (124) 1449 (10) 1445 (13) 1342 (81) 1377 (252) 1152 (57) 1078 (0) 1011 (6)

3139 (4) 3089 (10) 3026 (13) 2970 (5) 2153 (680) 1687 (186) 1467 (8) 1460 (11) 1366 (85) 1340 (184) 1137 (76) 1102 (1) 1016 (5)

exp 3095 (3)

2143 (100) 1681 (42) 1434- (31421 2) 1379 (41) 1345 (15) 1169 (6) 1015 (3)

MP2/6-31G*b

BLYP/6-31G*c

3092 (22) 3052 (3) 3006 (5) 2928 (2) 2096 (583) 1660 (236) 1453 (13) 1444 (8) 1363 (55) 1315 (106) 1208 (120) 1063 (8) 1008 (6)

3155 (12) 3086 (8) 3027 (10) 2972 (4) 2135 (621) 1696 (307) 1470 (9) 1457 (10) 1369 (21) 1320 (52) 1199 (156) 1094 (18) 1020 (6)

exp 3084 (2)

2133 (100) 1698 (54) 1434- (31421 2) 1365 (18) 1343 (16) 1221 (14) 1081 (2) 1021 (3)

assignmentd C-H stretch C-H (Me) asym stretch C-H (Me) asym stretch C-H (Me) sym stretch CdCdO stretch CdO stretch CH3 asym rock CH3 sym. rock C*-CMe stretch C*-CCO stretch C-C*-C bend CHdCdO bend CH3 torsion

a Frequency in cm-1 and calculated intensity in km mol-1; intensity values are given in parentheses. b Scaled by 0.9427 (ref 9). c Unscaled values. d C* denote the carbonyl carbon.

TABLE 6: Calculated and Experimental Infrared Spectra of Deuteroacetylketenes s-Z-1 and s-E-1a s-Z-1 ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13

s-E-1

MP2/6-31G*b

B-LYP/6-31G*c

2294 (8) 3048 (1) 3008 (1) 2929 (0 2099 (100) 1650 (23) 1449 (2) 1445 (2) 1368 (24) 1316 (36) 1135 (6) 1011 (0) 946 (1)

2319 (6) 3089 (2) 3026 (2) 2970 (1) 2145 (100) 1684 (30) 1467 (1) 1460 (2) 1363 (5) 1308 (35) 1130 (10) 908 (4) 1016 (1)

exp 2306 (4)

2137 (100) 1680 (48) 1439- (91426 8) 1364 (42) 1325 (32) 1155 (6) 937 (3)

-1

a

Frequency in cm and calculated intensity in km mol values. d C* denote the carbonyl carbon.

-1;

MP2/6-31G*b

B-LYP/6-31G*c

2287 (9) 3052 (1) 3006 (1) 2928 (1) 2088 (100) 1658 (42) 1453 (2) 1444 (2) 1363 (10) 1295 (27) 1171 (14) 1007 (0) 978 (1)

2331 (6) 3086 (1) 3027 (2) 2972 (1) 2127 (100) 1695 (50) 1470 (2) 1457 (2) 1368 (4) 1288 (15) 1170 (22) 891 (1) 1020 (1)

exp 2243 (4)

2127 (100) 1696 (63) 1439- (101426 11) 1356 (13) 1320 (35) 1194 (31) 993 (9)

assignmentd C-D stretch C-H (Me) asym stretch C-H (Me) asym stretch C-H (Me) sym stretch CdCdO stretch CdO stretch CH3 asym rock CH3 sym rock C*-CMe stretch C*-CCO stretch C-C*-C bend CDdCdO bend CH3 torsion

intensity values are given in parentheses. b Scaled by 0.9427 (ref 9). c Unscaled

TABLE 7: Calculated and Experimental Infrared Spectra of Trideuteroacetylketenes s-Z-2 and s-E-2a s-Z-2 MP2/6-31G* ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 a

b

3098 (12) 2261 (2) 2228 (3) 2105 (123) 2108 (481) 1646 (129) 1046 (5) 1043 (4) 1154 (77) 1362 (316) 1026 (25) 1078 (0) 870 (7)

BLYP/6-31G* 3139 (5) 2289 (4) 2239 (7) 2134 (2) 2153 (680) 1682 (192) 1059 (4) 1053 (7) 1125 (92) 1345 (257) 1102 (1) 1022 (43) 874 (7)

-1

s-E-2 c

exp

MP2/6-31G* 3092 (22) 2263 (1) 2226 (2) 2105 (1) 2096 (583) 1654 (242) 1049 (65) 1047 (5) 1218 (141) 1315 (130) 1035 (31) 1065 (4) 860 (9)

2144 (100) 1678 (33) 1034- (51053 3) 1345 (16) 1028 (6) 866 (4)

Frequency in cm and calculated intensity in km mol values. d C* denote the carbonyl carbon.

b

BLYP/6-31G* c 3155 (12) 2287 (4) 2240 (5) 2136 (230) 2134 (392) 1690 (317) 1060 (5) 1055 (7) 1198 (178) 1318 (48) 1040 (39) 1093 (18) 876 (9)

exp

2136 (96) 2135 (100) 1694 (28) 1034- (51053 3) 1209 (7) 1324 (11) 1034-1053 859 (4)

assignmentd C-H stretch C-D (Me) asym stretch C-D (Me) asym stretch C-D (Me) sym stretch CdCdO stretch CdO stretch CD3 asym rock CD3 sym rock C*-CMe stretch C*-CCO stretch C-C*-C bend CHdCdO bend CD3 tilt

-1

; intensity values are given in parentheses. b Scaled by 0.9427 (ref 9). c Unscaled

CD3sCOsCHdCdO (2) isotopomers. The assignment of all vibrational frequencies is given in Tables 5, 6, and 7. A significant frequency shifts is expected for vibrations involving the deuterium isotopes. This is readily seen in ν1, ν9, ν10, ν12, and ν13 vibrations for 1 and ν2, ν3, ν4, ν7, ν8, ν9, ν10, ν11, and ν13 vibrations for 2. MP2/6-31G* and B-LYP/631G* theories predict similar frequency shifts for compound 1 and 2. Significant intensity changes are calculated, e.g. for the C-D (ν4) and CdCdO (ν5) stretches for s-Z-2 at the MP2/6-31G* level, whereas the B-LYP/6-31G* method predicts the intensities of ν4 and ν5 to be essentially unchanged vis-a-vis the undeuterated acetylketene. The large intensity changes at the MP2/ 6-31G* level are due to the close coincidence of ν4 and ν5

vibrational frequencies. As a consequence, there is a significant mixing of the two vibrations. It is necessary to mention at this point that the quantitative evaluation of matrix IR spectra of isolated species has to be treated very carefully. Intensities can change unpredictably due to matrix site effects.11 Nevertheless, the predicted frequencies from MP2 calculations, even with the standard scaling factor of 0.9427, and those from B-LYP, especially in the fingerprint region, are a very useful tool for the assignment of the observed bands to particular conformers. We have considered two plausible 1,3-H migrations of acetylketene (11), leading to hydroxymethylmethyleneketene (12) and hydroxyvinylketene (13) (Scheme 3, QCISD(T)6-

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Zuhse et al.

SCHEME 2

SCHEME 3: Energy Values in kJ mol-1 Figure 1. Partial FTIR spectra (Ar, 14 K) of s-Z-1 and s-E-1 with a smaller amount of the corresponding protio compound 11 (b), produced by FVP of 3 (650 °C, 10-5 mbar, see Table 1): (a) after deposition (9, s-Z/E-1; b, protio compound), (b) after irradiation at 313 nm for 2h ([ indicating the new photosites), (c) after annealing to 35 K for 2 min, and (d) difference spectrum (b) - (a).

311+G(2d,p) + ZPVE level). 12 and 13 are predicted to lie 81 and 128 kJ mol-1 above acetylketene (11). Reversion of 12 to 11, via transition structure 14, requires an energy barrier of 126 kJ mol-1. Reversion of 13 to 11 requires a barrier of 142 kJ mol-1 (via transition structure 15). The calculated barriers for these 1,3-migration processes are very substantial compared to the small energy requirement inferred from the annealing experiments at 35 K (see below). Hence, it is unlikely that the new absorption bands are due to the isomeric structures 12 or 13. We have computed the IR spectra of two isotopomers of 12 (i.e., 7 and 9) and 13 (8 and 10). The calculated OH and OD stretching frequencies are summarized in Table 3. Irradiation. Monochromatic photolysis at 313 nm of an Ar matrix isolated mixture of the s-E and s-Z conformers of acetylketene produces a new set of absorptions. In the ketene region of the monodeuterated R-oxoketene 1 the major absorptions for the s-Z and s-E conformer are 2137 and 2127 cm-1 (Scheme 1, Table 1). The weaker bands at 2143 and 2133 cm-1 are due to the corresponding protio derivative (11), which is generated from 3 in the pyrolysis tube by contact with unremoved OH functions on the quartz surface. After 2.5 h of irradiation at 313 nm, a new set of bands could be observed at 2151, 2147, 2131, and 2123 cm-1. The photostationary state is reached after 2.5 h, and further irradiation causes no drastic change in the absorption pattern. Annealing of the sample to 35 K made the new bands disappear and a spectrum very similar to that before photolysis was regenerated. The previously minor bands at 2144 and 2131 cm-1 increase and the major ketene absorptions at 2137 and 2127 cm-1 decrease in intensity on photolysis. The band at 2144 cm-1 is unusually broad, which is a result of the overlap of this absorption with one of the major bands from the corresponding protio compound at 2143 cm-1. In both deuterated compounds the change in intensities from

Figure 2. Partial FTIR spectra (Ar, 14 K) of s-Z-2 and s-E-2 with a smaller amount of the perdeuterated compound 6, produced by FVP of 4 (450 °C, 10-5 mbar, see Table 1): (a) after deposition (9, s-Z/ E-2; 0, s-Z/E-6; b, C-D symmetry stretching band of s-E-2, see also Table 7), (b) after irradiation at 313 nm for 2 h ([ indicating the new photosites), (c) after annealing to 35 K for 2 min, and (d) difference spectrum: (b) - (a).

the “major” to the “minor” sites as a result of the photolysis/ annealing cycle is more pronounced for the E- than for the Z-conformer. The same observation was made for the undeuterated compound.6 The irradiation of s-E/Z-2 likewise gave a new set of bands. The major ketene absorptions are at 2144 cm -1 for the Z-conformer and at 2135 cm-1 for the E-conformer. After photolysis, new bands appear at 2151 and 2120 cm-1 (Table 1). Again, after annealing to 35 K the additional bands disappear and the minor ketene absorptions at 2140 and 2133 cm-1 increase in intensity. The absorptions at 2132 and 2125 cm-1 are due to the completely deuterated R-oxo-ketene (6) which results from the workup with D2O in the synthesis of the deuterated precursor 4, giving 5 (Scheme 1). The MP2 calculations predict that the ketene bands for 6 are at 2126 cm-1 (with the scaling factor 0.9600) for the s-E conformer and at 2137 cm-1 for the s-Z conformer. The expected photoinduced changes in the ketene absorptions are observed for all the deuterated compounds (1, 2, and 6; see Figures 1 and 2). The final decision whether these new bands belong to photoconformers or are due to completely new species generated

Photochemistry of Deuterated Acetylketenes by a 1,3-hydrogen shift upon irradiation at 313 nm can be made by producing difference spectra from the matrix-isolated conformer mixture before and after photolysis. Since the deuterated compounds would produce species like 7, s-Z/E-8 or s-Z/E-10, and 9 (Scheme 2) in the case of a 1,3-H-shift during photolysis, new OD absorptions would appear and the difference spectra would indicate them clearly. A Hooke’s law calculation predicts the OD-absorption to be in the region 2700-2500 cm-1, and the MP2/6-31G* calculations predict absorptions in the range 2600-2500 cm-1 for compounds 7 and 9 (Table 3). This region is free of other interfering absorptions. The absorption due to CH3OD (1042, 2704, 2845, 2955 cm-1) from the pyrolysis of 3 was completely compensated in the difference spectra. The actual difference spectra from the photolysis of 1 and 2 showed no traces of any new absorptions that could have been due to OD groups in the region 2160-2900 cm-1. Prolonged irradiation of the Z/E mixtures of the deuterated acetylketenes in xenon or nitrogen matrices shows alterations of the relative intensities due to s-E f s-Z conversion, but no easily identifiable changes ascribable to new photoisomers (Table 4. Spectra of the ketene regions are shown in Figures 3-6 in the supplementary material). This confirms the conclusion that the effect observed in Ar matrix is a site effect. Discussion The photochemical 1,3-shifts illustrated in Scheme 2 cannot be the reason for the drastic change in the Ar matrix IR spectrum of acetylketene. As described above, there are several experimental and theoretical arguments against the occurrence of such 1,3-hydrogen shifts. The strongest evidence comes from the photolysis of the deuterated species. Here, the 1,3-shift in 1 would lead to the hydroxy(methyl)methyleneketene 7 in which the hydroxy group is deuterated. The alternative enolization of 2 would produce 10, likewise possessing on OD group. The total absence of any new peak in the range 2160-2900 cm-1 rules out the possibility of a photochemically generated ODcontaining compound. Only in the ketene region and in particular areas in the fingerprint region of the IR spectra, new site bands appear next to the existing bands on irradiation at 313 nm. These new absorptions disappear again on annealing of the Ar matrix to 35 K. This is true for both the protio and the deuterated compounds. Thus, there is no discernible H/D kinetic isotope effect, and the occurrence of hydrogen shift isomers is extremely unlikely. Other experimental studies have shown that conformers with barriers higher than 6 kJ mol-1 can be trapped at 20 K, while annealing to 35 K leads to the loss of the less stable conformer for molecules with rotational barriers of less than 12 kJ mol-1.11c,12 In contrast, our ab initio calculations indicate that barriers above 100 kJ mol-1 would apply if OD tautomers were involved. Accordingly, the new bands cannot be due to new compounds but must be due to photochemically populated metastable sites which to a large extent revert to the lower energy sites on annealing. Conclusion Distinguishing absorptions arising from metastable matrix sites from those due to new molecules is an important problem in matrix isolation spectroscopy. Site effects, in many cases weak or undetectable, can lead to difficulties in the assignment and misinterpretations when they are strong and pronounced as in the case of acetylketene. By using deuterated precursors (3, 4) of acetylketene (1, 2, 6), it is demonstrated that new bands appearing in the ketene region in the Ar matrix IR spectra as a result of UV photolysis

J. Phys. Chem., Vol. 100, No. 10, 1996 3921 are not due to photochemically generated tautomers containing an OD group (7 or 10). The facile and almost complete reversion to the original spectra on annealing at 35 K indicates that the new bands are due to photoinduced sites in the Ar matrix. A similar photochemical site effect is not observed in N2 or Xe matrices, where only a photochemical s-E f s-Z rotamerization takes place. This work leaves open the question whether formylketene exhibits fundamentely different matrix photochemistry, tautomerizing to hydroxymethyleneketene,7 or whether the new bands observed here were in fact due to photoinduced metastable sites. As it is now evident that ketenes can show exceptionally strongsand potentially confusingsmatrix site effects, interpretations of ketene photochemistry in matrix have to be treated with great caution. For example, the postulated formation of dimethyloxirene,13 reported to absorb in the ketene region of the IR spectrum, and its photoisomerization to dimethylketene on either photolysis or annealing to 25 K are worthy of reinvestigation. Experimental and Computational Details Standard ab initio molecular orbital calculations14a were carried out with the GAUSSIAN 92/DFT14b series of programs. Geometry optimizations were performed with the polarized splitvalence 6-31G* basis set14a at the second-order Møller-Plesset perturbation (MP2) and the B-LYP15 levels. The B-LYP theory is a density functional method comprising the Becke’s parameter exchange function15a and the Lee-Yang-Parr (LYP)15b gradient-corrected representation of the correlation function. Harmonic vibrational frequencies and infrared intensities were predicted at these levels using analytical second derivatives. The directly calculated MP2 frequencies were scaled by 0.9427 to account for the overestimation of vibrational frequencies at this level of theory.9 Improved relative energies were obtained through quadratic configuration interaction with singles, doubles and augmented triples [QCISD(T)]16 calculations with the larger 6-311+G(2d,p)14a basis set, based on the MP2(full)/6-31G* optimized geometries. This level of theory is evaluated with the use of the additivity approximation,

∆E(QCISD(T)/6-311+G(2d,p)) ) ∆E(QCISD(T)/6-31G*) - ∆E(MP2/6-31G*) + ∆E(MP2/6-311+G(2d,p)) Our best relative energies correspond to QCISD(T)/6-311+G(2d,p) values with zero-point vibrational contributions. The frozen-core approximation was employed for all single-point correlated calculations. The FTIR spectroscopy was performed using a Perkin Elmer 1720-X FTIR spectrometer, normally accumulating 16 scans. The resolution used was 1 or 0.5 cm-1 in the spectral range 750-4000 cm-1. The FVP apparatus and cryostat were as previously reported.6 Samples were deposited on BaF2 disks and spectra recorded at 14 K. During deposition an operating pressure of 10-5 mbar or lower was maintained. The reference spectra of acetone, CH3OH, and CH3OD were taken under the same conditions at which they were generated upon the pyrolysis as byproducts of the acetylketenes. In the case of 3 as precursor, the ratio of absorptions at 1033 cm-1 for CH3OH and 1042 cm-1 for CH3OD was used as an indicator of the percentage of deuteration of the generated ketene. For matrix isolation, samples were sublimed (at -30 °C for 3, and 45-55 °C for 2) and cocondensed with Ar (5 mmol h-1) over 40-60 min period, thus ensuring a high matrix/substrate (M/S) ratio to avoid

3922 J. Phys. Chem., Vol. 100, No. 10, 1996 aggregation. Argon was of ultrahigh purity (99.999%, CIG). UV irradiation of the matrices was carried out with a 1000 W Hanovia high-pressure Xe-Hg lamp with a monochromator (Schoeffel GM 252). It was necessary in all cases to treat the pyrolysis apparatus and deposition lines with hexamethyldisilazane to prevent H/D exchange on the surface of the quartz tube during the experiment. Without such treatment, the undeuterated ketene was invariably the main constituent. Methyl 2,2-dideuterioacetylacetate (3) was generated by stirring the undeuterated compound with a 10-fold volume of D2O at room temperature for 2 h, followed by extraction with ether. This procedure was repeated three times to obtain a satisfactory percentage of deuteration (>95%). 1H NMR (CDCl3) δ 2.2 (s, 3 H), 3.7 (s, 3 H). The NMR spectrum showed a weak triplet at δ 3.45 due to the monodeuterated compound (less than 5% of 1 H). 5,5-Dideuterio-2,2-dimethyl-1,3-dioxane-4,6-dione17 was prepared by three recrystallizations of 500 mg of the undeuterated compound in 10 mL of D2O (99 atom %, Sigma) and 2 mL of acetone. 1H NMR (CDCl3) δ 1.8 (s, 6 H). The 1H NMR spectrum showed a weak triplet at δ 3.6 (3% of 1 H). 13C NMR (CDCl3) δ 27.5, 95.6, 106.2, 162.9. 2,2-Dimethyl-5-(2,2,2-trideuterio-1-hydroxyethylidene)1,3-dioxane-4,6-dione (4) was prepared by acylation18 of 5,5dideuterio-2,2-dimethyl-1,3-dioxane-4,6-dione with commercially available acetic acid chloride-d3 (99 atom %, Aldrich) following the procedure of Yamamoto et al.18b 1H NMR (CDCl3) δ 1.7 (s, 6 H). The quintet at 2.6 ppm showed a deuteration grade of >96%. 13C NMR (CDCl3) δ 26.8, 91.8, 104.9, 160.4, 170.1, 194.5. Acknowledgment. We thank the Australian Research Council (ARC) for financial support (to C.W.) and for a Research Fellowship for M.W.W., and The University of Queensland for generous allocation of supercomputer time. Supporting Information Available: Table of calculated MP2/6-31G* frequencies and intensities of hydroxy(methyl)methyleneketenes (7, 9, 12) and the conformers of the hydroxyvinylketenes (s-Z/E-8, s-Z/E-10, s-Z/E-13, Table 8). Partial IR spectra showing the ketene regions of s-Z/E-1 and s-Z/E-2 in Xe and N2 matrices at 14 K, with the results of irradiation at 313 nm (Figures 3-6). This material (7 pages) is contained in libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. References and Notes (1) (a) Review: Wentrup, C.; Heilmayer, W.; Kollenz, G. Synthesis 1994, 1218-1248; (b) Tidwell, T. T. Ketenes; Wiley: New York, 1995. (2) (a) Clemens, R.; Witzeman, J. S. J. Am. Chem. Soc. 1989, 111, 2186. (b) Witzeman, J. S. Tetrahedron Lett. 1990, 31, 1401. (c) Allen, A. D.; Andraos, J.; Kresge, A. J.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc. 1992, 114, 1878. (d) Winnik, M. A.; Wang, F.; Nivaggioli, T.; Hruska, Z. J. Am. Chem. Soc. 1991, 113, 9702.

Zuhse et al. (3) (a) Emerson, D. W.; Titus, R. L.; Gonzalez, R. M. J. Org. Chem. 1991, 56, 5301. (b) Hyatt, J. J. Org. Chem. 1984, 49, 5102. (c) Maier, H.; Wengenroth, H.; Lauer, W.; Krause, V. Tetrahedron Lett. 1989, 30, 5253 and references cited therein. (4) (a) Freiermuth, B.; Wentrup, C. J. Org. Chem. 1991, 56, 2286. (b) Leung-Toung, R.; Wentrup, C. J. Org. Chem. 1992, 57, 4850. (c) LeungToung, R.; Wentrup, C. Tetrahedron 1992, 48, 7641. (d) Kappe, C. O.; Evans, R. A.; Kennard, C. H. L.; Wentrup, C. J. Am. Chem. Soc. 1991, 113, 4234. (e) Andreichikov, Yu. S.; Kollenz, G.; Kappe, C. O.; LeungToung, R.; Wentrup, C. Acta Chem. Scand. 1992, 46, 683. (f) Kappe, C. O.; Fa¨rber, G.; Kappe, T.; Wentrup, C. Chem. Ber. 1993, 126, 2357. (g) Wentrup, C.; Netsch, K.-P. Angew. Chem., Int. Ed. Engl. 1984, 23, 802. (h) Wentrup, C.; Winter, H.-W.; Gross, G.; Netsch, K.-P.; Kollenz, G.; Ott, W.; Biederman, A. G. Angew. Chem., Int. Ed. Engl. 1984, 23, 800. (i) McMahon, R, J.; Chapman, O. L.; Hayes, R. A.; Hess, T. C.; Krimmer, H.-P. J. Am. Chem. Soc. 1985, 107, 7597. (j) Chapman, O. L.; Miller, M. D.; Pitzenberger, S. M. J. Am. Chem. Soc. 1987, 109, 6867; (k) Murata, S.; Yamamoto, T; Tomioka, H. J. Am. Chem. Soc. 1993, 115, 4013. (5) (a) Janoschek, R.; Fabian, W. M. F.; Kollenz, G.; Kappe, C. O. J. Comput. Chem. 1994, 15, 132. (b) Birney, D. M. J. Org. Chem. 1994, 59, 2557. (c) Wagenseller, P. E.; Birney, D. M.; Roy, D. J. Org. Chem. 1995, 60, 2853. (d) Wong, M. W.; Wentrup, C. J. Org. Chem. 1994, 59, 5279. (6) Kappe, C. O.; Wong, M. W.; Wentrup, C. J. Org. Chem. 1995, 60, 1686. (7) Maier, G.; Reisenauer, H.-P.; Sayrac, T. Chem. Ber. 1982, 115, 2192. (8) For unsubstituted deuterated ketene see: (a) Arendale, W. F.; Fletcher, W. H. J. Chem. Phys. 1957, 26, 793. (b) Moore, C. B.; Pimentel, G. C. J. Chem. Phys. 1962, 38, 2816. (c) Duncan, J. L.; Ferguson, A. M.; Harper, J.; Tonge, K. H. J. Mol. Spectrosc. 1987, 122, 72. (9) Pople, J. A.; Scott, A. P.; Wong, M. W.; Radom, L. Isr. J. Chem. 1993, 33, 345. (10) For reviews, see: (a) Ziegler, T. Chem. ReV. 1991, 91, 651. (b) Density Functional Methods; Labanowski, J., Andzelm, J., Eds.; Springer: Berlin, 1991. (c) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1992, 96, 1280. (11) For a detailed discussion of matrix effects see: (a) Barnes, A. J. In Matrix Isolation Spectroscopy; Barnes, A. J., Orville-Thomas, W. J., Mu¨ller, A., Gaufre´s, R., Eds.; D. Reidel: Dordrecht, 1981; pp 13-26. (b) Cradock, S.; Hinchcliffe, A. J. Matrix Isolation; Cambridge University Press: Cambridge, 1975; pp 5-25, and pp 88-102. (c) Barnes, A. J. J. Mol. Struct. 1984, 113, 161. (12) (a) Gebicki, J.; Plonka, A.; Krantz, A. J. Chem. Soc., Perkin Trans. 2 1990, 2051. (b) Kulbida, A.; Fausto, R. J. Chem. Soc., Faraday Trans. 1993, 89, 4257. (c) Kulbida, A.; Nosov, A. J. Mol. Struct. 1992, 265, 17. (d) Gatial, A.; Klaeboe, P.; Nielsen, C. J.; Sablinskas, V.; Powell, D. L.; Kondow, A. J.; Incavo, J. A. J. Mol. Struct. 1993, 295, 73. (e) El-Bindary, A. A.; Horn, A.; Klaeboe, P.; Nielsen, C. J. J. Chim. Phys. 1993, 90, 1685. (13) Bachmann, C.; N’Guessan, T. Y.; Debu, F.; Monnier, M.; Pourcin, J.; Aycard, J. P.; Bodot, H. J. Am. Chem. Soc. 1990, 112, 7488. (14) (a) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (b) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92/DFT; Gaussian Inc.: Pittsburgh PA, 1992. (15) (a) Becke, D. A. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (16) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. (17) (a) cf. Meldrum, A. N, J. Chem. Soc. 1908, 93, 598 (b) Davidson, D.; Bernhard, S. A. J. Am. Chem. Soc. 1948, 70, 3426. (18) (a) Oikawa, Y; Sugano, K; Yonemitsu, O. J. Org. Chem. 1978, 43, 2087. (b) Yamamoto, Y; Watanobe, Y; Ohnishi, S. Chem. Pharm. Bull. 1987, 35 (5), 1860.

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