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A Molecular Dynamics Study of Pressure Effects on Solvation and Optical Spectra: the Ground and Excited States of Formaldehyde in Water

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A Molecular Dynamics Study of Pressure Effects on Solvation and Optical Spectra: the Ground and Excited States of Formaldehyde in Water 1082 J. Phys. Chem. 1991, 95, 1082-1089 albedo isj = 12.6 X IO4 s-l, 17% (24%) lower than our value. form progressions of the upper state N4stretching vibration, 02’. Deconvolution of the spectrum into progressions of the trans 5. Summary and Conclusions and cis rotamers yields an estimate of the equilibrium constant A UV absorption spectrum of pure gaseous HONO has been N,,,,,/Nci, = K, corresponding to an energy difference AE = -646 obtained at a spectral resolution of 0.1 nm, by careful subtraction cal mo1-I (2700 J mol-I) between the rotamers, in good agreement of NO2 from composite spectra of highly diluted HONO/NO/ with a recent theoretical estimate. N02/H20mixtures in nitrogen. Existing uncertainties in absolute absorption cross sections could be reduced to approximately f5% Acknowledgment. This work, part of which has been presented or f5 X IC2’cm2, whichever is greater, in the investigated spectral at the EUROTRAC Symposium 1990 in Garmisch-Partenkirchen, range. Owing to recent theoretical work, the photochemistry of was supported by the European Economic Community and by the HONO, particularly the continuous nature of the spectrum and German BMFT. A financial contribution of Fonds der Chem- the line shapes of bands, are now well established. Salient features ischen Industrie to our computing and word processing system of the spectrum have been reproduced by a synthetic spectrum is gratefully acknowledged. We thank Silke Schweighoefer for composed of Lorentzian bands of the trans and cis rotamers, which valuable assistance. A Molecular Dynamics Study of Pressure Effects on Solvation and Optical Spectra: The Ground and Excited States of Formaldehyde in Water Mahfoud Belhadj, Douglas B. Kitchen, Karsten Krogh-Jespersen, and Ronald M. Levy* Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08904 #(Received:July 23, 1990) Molecular dynamics simulations of formaldehyde in liquid water have been carried out at high pressures (1 and IO kbar). The results of three sets of simulations are reported corresponding to the equilibrium solvation of this model dipolar solute in the ground state (IAl), the first excited singlet state (‘A”), and the nonequilibrium solvation following a simulated electronic excitation. The high-pressure simulations are compared with the results at low pressure (1 bar) reported previously. The changes in the structure of the solvation shell of the ground-state species as the pressure is increased to 10 kbar are relatively small. In contrast, there are more substantial changes in the solvation structure of the excited-state species with increasing pressure. At low pressure, the solvation shell surrounding the ’A” state is disordered relative to that of the ground state. High pressure induces the formation of a distinct solvation shell around the formyl oxygen in the excited state. The waters in the first solvation shell have rotated by 180°, however, with respect to their preferred orientation in the ground state. Because of compensating effects involving different atoms of the solute, the changes in the total solvation energies of the ground and excited states with pressure are small, leading to a small pressure-induced blue shift in the optical absorption. These results indicate that it is important to account for the extended shape and charge distribution of the solute and the molecular nature of the solvation shell when considering pressure-induced effects on optical spectra. The nonequilibrium solvation dynamics corresponding to a laser-induced excitation is simulated by following 80 trajectories which start from different configurations on the ground-state surface. At high pressure, the rotational relaxation of waters in the first solvation shell is an order of magnitude slower than the solvent response at low pressure. The use of simulations to aid in developing Publication Date: February 1, 1991 | doi: 10.1021/j100156a013 molecular models for “pressure tuning spectroscopy” experiments is discussed. Downloaded by RUTGERS UNIV on October 22, 2009 | http://pubs.acs.org Introduction response following an optical excitation.’-15 We have been en- The analysis of solvent effects on optical absorption bands is gaged in a series of calculations on the effects of solvation on the a classical method that has long been used to study the energetics electronic and spectral properties of model solute-solvent sys- and dynamics of s~lvation.~-~In the simplest description of the tem~.~~~~In a recent communication, we reported the results of process, an optical excitation leads to a sudden charge redistri- molecular dynamics simulations of the time-resolved fluorescence bution within the solute; the differential solvation between the from the first excited singlet state (’A’’) of a model dipolar solute, two electronic states results in an absorption line shift. After formaldehyde, in water.14dIn this paper we extend this work to excitation, the solute and solvent begin to relax and this leads to simulations of high-pressure effects on the solvation of the ground a time-dependent fluorescence shift. The intimate relationship that exists-between effects observed in time-resolved fluorescence experiments and electron transfer has been discussed by several (7) Wolynes, P. G. J. Chem. Phys. 1987, 86, 5133. (8) Loring, R.; Yan. Y.; Mukamel, S. J. Chem. Phys. 1987, 87, 5841. Several lines of theoretical analysis point to the (9) Nichols, A.; Calef, D. J. Chem. Phys. 1988, 89, 3783. important role played by the first solvation shell in the solvent (IO) Karim, 0.;Haymet, A. D.; Banet, M. J.; Simon, J. D. J. Phys. Chem. 1988. 92. 3391. (1’1) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1988, 89, 5044. (I) Bayliss, N. J. J. Chem. Phys. 1950, 18. 292. (12) Maroncelli, M.; Maclnnis, J.; Fleming, G. Science 1989, 24, 1674. (2) Marcus, R. A. J. Chem. Phys. 1965, 43, 1261. (13) Chandra, A.; Bagchi. B. J. Phys. Chem. 1990, 94, 1874. (3) Okamoto, B. Y.; Drickamer, H. G.Proc. Natl. Acad. Sci. U.S.A. 1974, (14) (a) Blair, J.T.; Westbrook, J. D.; Levy, R. M.; Krogh-Jespersen, K. 71, 2671. Chem. Phys. Lett. 1989, 154, 531. (b) Blair, J. T.; Krogh-Jespersen, K.; Levy, (4) Mataga, N.; Kubota, T. Molecular Interactions and Electronic R. M. J. Am. Chem. Soc. 1989, 111, 6948. (c) Blair, J. T.; Levy, R. M.; Specfra; Marcel Dekker: New York, 1970. Krogh-Jespersen. K. Chem. Phys. Lett. 1990, 166, 429. (d) Levy, R. M.; (5) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Kitchen, D. B.; Blair, J. T.; Krogh-Jespersen, K. J. Phys. Chem. 1990, 94, Oxford University Press: New York, 1986. 4470. (6) Van der Zwan, G.; Hynes, J. T. J. Phys. Chem. 1985.89, 4181. (I 5) Debolt, s. E.; Kollman, P. A. J. Am. Chem. Soc., in press. 0022-3654/9 1 /2095- 1082$02.SO/O 0 199 I American Chemical Society MD Simulations of Formaldehyde The Journal of Physical Chemistry, Vol. 95, NO. 3, 1991 1083 and excited states of formaldehyde. Pressure-dependent studies simulations at three different pressures, 1 bar, 1 kbar, and 10 kbar, of the optical spectrum of dissolved solutes can, in principle, provide are reported. Each simulation was continued for 20 ps. The a great deal of information about electronic structure and so- average temperature in these simulations was 298 K. The mo- lutesolvent interactions.16 Simulations can have a valuable role lecular mechanics parameters employed for ground and excited in separating intramolecular from intermolecular effects. Mo- states of formaldehyde were the same as those used in our previous lecular interpretations of pressure-dependent spectral changes in work.Ik For water, the SPC model was used.26 The average water are particularly difficult; large pressure-induced red shifts volume of the simulation box was 6.49 X IO4 A3 at 1 bar, 6.29 have been attributed to attractive intermolecular forces between X lo4 A3 at 1 kbar, and 5.17 X IO4 A3 at 10 kbar. This corre- solute and solvent molecules participating in hydrogen bonds." sponds to a -25% increase in density as the pressure is increased Available evidence from simulations suggests that, for neat liquid from 1 bar to IO kbar; the results are in reasonable accord with water at least, the effect of pressure (at 10 kbar) is actually to experiment for which the observed increase is 22% at 340 K.27 decrease the strength of individual hydrogen bonds between water The equilibrium geometry of ground state (So)formaldehyde molecules, while the number of hydrogen bonds to each water is planar (C2"symmetry, ]Al),whereas the structurally relaxed, molecule increases.I8 For liquid methanol, the application of excited singlet state (SI)is pyramidal around the C atom (C, pressures to I5 kbar does not appear to increase the strength or symmetry, IA"). Separate equilibrium molecular dynamics sim- number of hydrogen bonds.Ig For model solutes like formaldehyde ulations with formaldehyde in the ground and excited state have which undergo a large change in their dipole moment and hy- been completed at 1 and 10 kbar. Solute-solvent radial and drogen-bonding properties upon excitation, it is unclear how angular distribution functions and solvation energies have been pressures will affect the relative solvation by water of the ground computed from each of the equilibrium simulations. In addition, versus the excited state and the hydrogen bonding between solute high-pressure nonequilibrium solvation simulations have been and solvent. carried out using the method described previously.'" The vertical Formaldehyde serves as a prototypical molecule to study the excitation of formaldehyde is simulated by instantaneously solvation and spectroscopic properties of the carbonyl group. The switching the formaldehyde charges and force constants from 'Al n - K* transition of the carbonyl group involves the promotion ground-state values to those corresponding to the IA" excited state. of an electron from an in-plane oxygen lone pair, oriented per- The system is then propagated forward in time by integrating the pendicular to the C=O axis and almost exclusively of O(2p) equations of motion.
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