Characterization and Photoisomerization of m-Xylylene
A R T I C L E S
Figure 9. (U)B3LYP optimized geometries of T-1, 7, 9, 11 and 12. Bond length is given in pm, and bond angle, in deg.
Table 5. DFT Energies of the C8H8 Isomers
h the solvent was removed, 150 mL of dichloromethane were added,
and the precipitate was filtered off. Removal of the solvent under
reduced pressure gave 1,3-bis-iodomethyl-benzene-d4 d4-8, which was
further purified by column chromatography using dichloromethane and
hexane (1:2) as eluent. Yield: 80%. IR (KBr) 2174, 1581, 1480, 1426,
1183, 1058, 926, 851,783, 690, 505 cm-1; mass spectrum, m/z 362
(M+).
Matrix Isolation. Matrix isolation experiments were performed by
standard techniques25 using a closed cycle helium cryostat and a CsI
spectroscopic window cooled to 10 K. FTIR spectra were recorded
with a standard resolution of 0.5 cm-1, using a N2(l)-cooled MCT
detector in the range 400-4000 cm-1. X-band EPR spectra were
recorded from a sample deposited on an oxygen-free high-conductivity
copper rod (75 mm length, 2 mm diameter) cooled with a closed cycle
cryostat to 4 K. UV spectra were recorded in the spectroscopic range
500 to 200 nm with a standard resolution of 0.02 nm, using a Varian
UV-vis NIR spectrophotometer from a sample deposited on a sapphire
window cooled to 10 K by a closed cycle cryostat. Flash vacuum
pyrolysis was carried out by slowly subliming 8 through a 7 cm quartz
tube heated electrically with a tantalum wire.
Broadband irradiation was carried out with mercury high-pressure
arc lamps in housings equipped with quartz optics and dichroic mirrors
in combination with cutoff filters (50% transmission at the wavelength
specified). For 254 nm irradiation a low-pressure mercury arc lamp
was used.
Computational Methods. Optimized geometries and vibrational
frequencies of all species were calculated at the B3LYP26-28 level of
theory employing the 6-311G(d,p) polarized valence-triple-ê basis
set.29,30 Tight convergence criteria were used throughout. A spin-
molecule
(E
+
ZPC)a hartrees
E
rel, kcal/mol
T-1b
S-1b
7c
-309.534386
-309.524472
-309.503791
-309.493361
-309.456873
-309.491230
0
6.22
19.20
25.74
48.64
27.08
9c
11c
12c
a All energies including zero-point vibrational energy correction (ZPC)
b
c
without spin-projection. UB3LYP/6-311G(d,p). B3LYP/6-311G(d,p).
Experimental Section
1,3-Bis-iodomethyl-benzene 8. Compound 8 was synthesized ac-
cording to a literature procedure.24
1,3-Bis-(hydroxymethyl)-benzene-d4. To a refluxing and stirred
suspension of lithiumaluminiumdeuterid LAD (0.60 g, 14.3 mmol) in
anhydrous THF (20 mL) under argon a solution of dimethyl benzene-
1,3-dicarboxylate (1.29 g, 6.65 mmol) in 10 mL of anhydrous THF
was added dropwise. The mixture was refluxed for 5 h and cooled
with ice. The excess LAD was hydrolyzed by careful addition of
water (2 mL), 15% aq. NaOH (2 mL), and again water (5 mL). After
stirring overnight, the white solid was filtered off and washed with
several portions of THF. All organic extracts were combined, and the
solvent was removed in vacuum to give a colorless oil which solidified
rapidly. The crude product was used without further purification in
the next step. Yield: 85%. IR (KBr) 3214, 2213, 2090, 1432, 1266,
1187, 1107, 1082, 1059, 954, 723, 690 cm-1; mass spectrum, m/z
142 (M+).
1,3-Bis-bromomethyl-benzene-d4. To a stirred solution of 0.90 g
(6.33 mmol) 1,3-bis-hydroxymethyl-benzene-d4 in 30 mL of anhydrous
THF a solution of 0.77 mL (2.23 g, 8.23 mmol) phosphorus tribromide
in 15 mL of dry ether was added dropwise and stirred for 4 h at room
temperature. 30 mL of water were added, and the organic layer was
washed two times with 15 mL of water. After drying of the organic
layer over MgSO4 and evaporation of the solvent, a colorless oil was
obtained which was purified by column chromatography (dichlo-
romethane/hexane, 1:1). Yield: 75%. IR (KBr) 2274, 2179, 1483, 1427,
1186, 1064, 953, 867, 791, 694 cm-1; mass spectrum, m/e 268 (M+).
1,3-Bis-iodomethyl-benzene-d4. 0.106 g (0.40 mmol) of 1,3-bis-
bromomethyl-benzene-d4 was dissolved in 10 mL of dry acetone, and
0.6 g of NaI (4.0 mmol) was added. After stirring under reflux for 12
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