Observation of Steric Effect in o-Methyl Benzoate - American ...


Observation of Steric Effect in o-Methyl Benzoate - American ...

11400 J. Phys. Chem. A, Vol. 109, No. 50, 2005 Woo et al.

hydrogen bonding, the ∠CHO angles are much smaller than

that expected for C-H‚‚‚O hydrogen bonding. This result shows

that not every close C-H‚‚‚O contact can form a hydrogen

bond. In fact, sometimes close C-H‚‚‚O contact can indeed be

repulsive, which appears to be the case in o-CH 3 C 6 H 4 CO 2 - .

6. Conclusions

Temperature-dependent and vibrationally resolved photoelectron

spectra of benzoate and the three methyl-substituted

benzoate anions are reported. Photodetachment features from

O lone pairs of the carboxylate and the ring π electrons were

observed. The temperature-dependent photoelectron spectra

showed that all four anions give similar spectral patterns;

CH 3 C 6 H 4 CO - 2 , m-CH 3 C 6 H 4 CO 2- , and p-CH 3 C 6 H 4 CO 2- yielded

well-resolved vibrational structures in the O lone pair region at

low temperatures, except for o-CH 3 C 6 H 4 CO - 2 , which yielded

broad spectra even at the lowest ion trap temperature. The

ground-state transition in all four species was observed to be

broad and poorly resolved, suggesting a large geometry change

between the anions and the neutral ground states. Theoretical

calculations showed that there is a large ∠OCO bond angle

reduction of ∼19° from the anion to neutral ground states in

all four species, consistent with the broad spectral features for

the ground-state transitions. Strong steric effect was found

between the methyl group and the carboxylate group in

o-CH 3 C 6 H 4 CO 2- , such that the -CO 2- group is pushed out of

the plane of the benzene ring by about 25° and its internal

rotational barrier is also significantly reduced. The reduction

of the rotational barrier makes o-CH 3 C 6 H 4 CO - 2 much more

difficult to be vibrationally cooled. And the strong coupling of

the CO 2 torsional mode and the OCO bending mode yielded

the broad spectra for the ortho-isomer. The present study shows

that there is no C-H‚‚‚O hydrogen bond in o-CH 3 C 6 H 4 CO - 2 .

Acknowledgment. This work was supported by the U.S.

Department of Energy (DOE), Office of Basic Energy Sciences,

Chemical Science Division. The experiment and calculations

were performed at the W. R. Wiley Environmental Molecular

Sciences Laboratory, a national scientific user facility sponsored

by DOE’s Office of Biological and Environmental Research

and located at Pacific Northwest National Laboratory, which is

operated for DOE by Battelle.

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