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Solvent effects in emission spectroscopy: A Monte Carlo quantum mechanics study of the n←π* shift of formaldehyde in water

Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction (INDO/CIS) are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 A from the center of mass of formaldehyde, showing the very long-range effects ...

From hydrogen bond to bulk: Solvation analysis of then-?* transition of formaldehyde in water

Supermolecular calculations that treat both the solute and the solvent quantum mechanically are performed to analyze the n- transition of formaldehyde in water. The liquid structures are generated by canonical (constant volume, temperature, and number of particles) (NVT) Metropolis Monte Carlo simulation. Autocorrelation function is calculated to obtain efficient ensemble average. Full quantum mechanical intermediate neglect of differential overlap/singly excited configuration interaction (INDO/CIS) calculations are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The largest cluster, corresponding to the third solvation shell, includes 1 formaldehyde and 80 water molecules. INDO/CIS calculations are performed on a properly antisymmetric reference ground-state wave function involving all valence electrons. The results are then extrapolated to the bulk limit. The estimated limit value for the solvatochromic shift of the n- transition of formaldehyde in water, compared to gas phase, is 2200 cm 1 . c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 192-198, 2000

Ab initio calculation of hydrogen bonds in liquids: A sequential Monte Carlo quantum mechanics study of pyridine in water

A systematic procedure based on the sequential Monte Carlo quantum mechanics (S-MC/QM) methodology has been used to obtain hydrogen bond strength and structures in liquids. The system considered is pyridine in water. The structures are generated by NVT Monte Carlo simulation, of one pyridine molecule and 400 water molecules. The hydrogen bonds are obtained using a geometric and energetic procedure. Detailed analysis shows that 62% of the configurations have one hydrogen bond. In the average, pyridine in liquid water makes 1.1 hydrogen bonds. The sampling of the structures for the quantum mechanical calculations is made using the interval of statistical correlation obtained by the autocorrelation function of the energy. A detailed statistical analysis is presented and converged results are obtained. The QM calculations are performed at the ab initio MP2/6-31+G(d) level and the results are compared with the optimized 1:1 cluster. Our results using QM calculations on 155 structures making one hydrogen bond gives an average binding energy of 3.7 kcal/mol, after correcting for basis set superposition error, indicating that in the liquid the binding energy is about 2/3 of the corresponding binding in the optimized cluster.

The sequential Monte Carlo-quantum mechanics methodology. Application to the solvent effects in the Stokes shift of acetone in water

Abstract The sequential Monte Carlo quantum mechanics methodology is used to obtain the solvent effects on the Stokes shift of acetone in water. One of the great advantages of this methodology is that all the important statistical information is known before running into the costly quantum mechanical calculations. This advantage is discussed not only with respect to the statistical correlation between the different structures generated by the simulation but also in the proper identification of hydrogen bonds in liquids. To obtain the solvent effects in the Stokes shift of the n–π ∗ absorption transition of acetone in water, quantum-mechanical calculations are performed in super-molecular structures generated by NVT Monte Carlo simulation. The statistical correlation between configurations is analyzed using the auto-correlation function of the energy. The largest calculations include one acetone and 170 water molecules. One-hundred INDO/CIS super-molecular calculations are performed for each solvation shell to obtain the statistical average value. The calculated solvatochromic shift of the n–π ∗ absorption transition of acetone in water, compared to gas phase, is ∼1310 cm −1 in good agreement with the experimental blue shift of 1500±200 cm −1 . For the emission of the relaxed excited state, the calculated shift is ∼1850 cm −1 . The total calculated solvent effect on the Stokes shift of acetone in aqueous solution is thus 540 cm −1 . A detailed analysis of the sampling of the configurations obtained in the Monte Carlo simulation is made and it is shown that all results represent statistically converged values.

Electronic polarization of liquid water: converged Monte Carlo-quantum mechanics results for the multipole moments

Sequential Monte Carlo/Quantum Mechanical (S-MC/QM) calculations of the dipole moment of liquid water using extensive and different quantum chemical methods and statistically converged results give an induced dipole moment of 0:74 � 0:14 D. This corresponds to a dipole moment of liquid water of 2:60 � 0:14 D, in excellent agreement with the value derived from the dielectric constant and other previous theoretical estimates. Change in multipole moments are also reported using statistically converged MP2/aug-cc-pVQZ calculations. 2003 Elsevier Science B.V. All rights reserved.

New developments in Monte Carlo/quantum mechanics methodology. The solvatochromism of β-carotene in different solvents

Abstract The solvatochromic shifts of the π - π ∗ transition of all- trans -β-carotene in isopentane, acetone, methanol and acetonitrile are studied using a sequential Monte Carlo/quantum mechanics (S-MC/QM) methodology. These different solvents are examples of systems of varied nature, differing in dielectric constants and covering a wide range of polarities, and including also polar and non-polar solvents. In S-MC/QM we first generate the structure of the liquid using Metropolis MC simulation and then perform the QM calculations in statistically uncorrelated configurations. It is shown that, in these cases, including only 40 QM calculations gives statistically converged results. To deal with elongated solutes the box of the MC simulation has been extended to a large rectangular shape. Then, a nearest-neighbor distribution function has been developed and generalizes the concept of solvation shells for a solute of any arbitrary shape. The calculated results are converged with respect to the number of solvent molecules that are included according to the nearest-neighbor distribution function. The results are found to be in very good quantitative and qualitative agreement with experiment. The dipole moments of the ground and excited π - π ∗ states of β-carotene are both zero and the transition shifts are thus dominated by the dispersive interaction. The inclusion of dispersion interaction in energy differences is then discussed.

The electronic spectrum of N-methylacetamide in aqueous solution: a sequential Monte Carlo/quantum mechanical study

Abstract Sequential Monte Carlo/quantum mechanical (S-MC/QM) calculations are performed to study the solvent effects on the electronic transitions of N -methylacetamide (NMA) in aqueous solution. Full quantum mechanical INDO/CIS calculations are performed in the super-molecular clusters generated by Monte Carlo (MC) simulation. The largest calculation involves the ensemble average of 75 quantum mechanical results obtained with the NMA solute surrounded by 150 water solvent molecules. After extrapolation to the bulk limit we find that the n → π * transition suffers a blue shift of 1755 cm −1 upon solvation and the π → π * transition undergoes a red shift of 1180 cm −1 , in good agreement with the experimental findings.

Conformational stability of furfural in aqueous solution: the role of hydrogen bonding

The importance of hydration on the equilibrium involving internal rotation around carbon-carbon single bond is investigated for the furfural molecule dissolved in water. To analyze the solvent effects in stabilizing any preferred conformation of furfural, we perform Monte Carlo (MC) NPT simulations corresponding to different rotation angles of the carbonyl group. The hydrogen bonds formed between solute and solvent are also analyzed along the conformational equilibrium. They are found to be equivalent, both in number and in binding energy, for all rotation angles. These results give a strong evidence that the conformational stability and the rotation barrier of the furfural molecule in water relate to the bulk properties rater than solute-solvent hydrogen bonds.

Van der Waals Interaction Probed by Solvatochromic Shifts

The study of molecular systems in the liquid phase is important for understanding a large number of chemical, physical and biological processes. The solvent interaction leads to changes in the molecular solute affecting its spectroscopic, structural and reactive properties. For this reason, the study of solvent effects has been a topic of increased interest1,2. UV-Vis absorption spectrum is very sensitive to solvent effects and it can thus be used judiciously in modeling intermolecular interaction. In this context van der Waals interactions can also be probed in the study of solvatochromic shifts. In particular for the case of nonpolar molecules in nonpolar medium the only remaining term of the dipolar van der Waals interaction is the so-called London dispersion term3. Dispersion is the mutual induced polarisation between the two sub-systems. Liptay4 has shown that dispersion interaction between the chromophore and the solvent contributes to a negative, or red, shift.

Hydrogen Bonding and the Energetics of Homolytic Dissociation in Solution

We discuss recent applications of microsolvation modeling and statistical mechanics Monte Carlo simulations to the energetics of homolytic bond dissociation in solution. This process, which generates free radical species in liquid phase is ubiquitous in many reactions of fundamental interest in chemistry and biochemistry including green plant photosynthesis and biocatalysis. Basic concepts of the microsolvation and Monte Carlo approaches to the energetics of homolytic bond dissociation in solution are reviewed and fundamentals of the sequential Monte Carlo/Quantum mechanics methodology are presented. By carrying out sequential Monte Carlo/Quantum mechanics calculations we stress the limitations of gas phase calculations based on local models such as microsolvation or hydrogen-bond-only and the importance of the statistical analysis of the solvation process for the understanding of the homolytic bond cleavage in solution. We give special emphasis to the relationship between solutesolvent hydrogen bonding, long-ranged interactions in polar solvents, and the energetics of homolytic bond dissociation.