On the Different Characteristics of Medium-Sized van der Waals Molecules Obtainable from ab initio Calculations

Nonempirical ab initio calculations with inclusion of the correlation energy are used to generate, directly or indirectly (via an analytical form of the potential-energy surface), various properties of medium-sized van der Waals molecules. As shown for benzene- • -Ar and the benzene dimer, the calculated characteristics agree with the experimental results. If the experimental values are lacking or if they are uncertain, the theoretical characteristics can be used with confidence

non covalent interactions in anisole co2 n n 1 2 complexes

Non-covalent interactions are a ubiquitous binding motif and a challenge for theory and experiments

Spectroscopy of Isolated Prebiotic Nucleobases

We use multiphoton ionization and double resonance spectroscopy to study the excited state dynamics of biologically relevant molecules as well as prebiotic nucleobases, isolated in the gas phase. Molecules that are biologically relevant to life today tend to exhibit short excited state lifetimes compared to similar but non-biologically relevant analogs. The mechanism is internal conversion, which may help protect the biologically active molecules from UV damage. This process is governed by conical intersections that depend very strongly on molecular structure. Therefore we have studied purines and pyrimidines with systematic variations of structure, including substitutions, tautomeric forms, and cluster structures that represent different base pair binding motifs. These structural variations also include possible alternate base pairs that may shed light on prebiotic chemistry. With this in mind we have begun to probe the ultrafast dynamics of molecules that exhibit very short excited states and search for evidence of internal conversions

The impact of the solvent dielectric constant on A←NH3 dative bond depends on the nature of the Lewis electron-pair systems

The present work aims to determine to what extent the value of the dielectric constant of the solvent can influence the dative bond in Lewis electron pair bonding systems. For this purpose, two different systems, namely H3B <- NH3 and {Zn <-(NH3)}(2+), were studied in selected solvents with significantly different dielectric constants. Based on the results from state-of-the-art computational methods using DFT, constrained DFT, energy decomposition analyses, solvent accessible surface area, and charge transfer calculations, we found that the stability of the neutral H3B <- NH3 system increases with increasing solvent polarity. In contrast, the opposite trend is observed for the positively charged {Zn <-(NH3)}(2+). The observed changes are attributed to different charge redistributions in neutral and charged complexes, which are reflected by a different response to the solvent and are quantified by changes in solvation energies.Web of Science293

Hydrogen bonding with hydridic hydrogen - Experimental low temperature IR and computational study: Is a revised definition of hydrogen bonding appropriate?

Spectroscopic characteristics of Me3Si−H···Y complexes (Y = ICF3, BrCN, and HCN) containing a hydridic hydrogen were determined experimentally by low-temperature IR experiments based on the direct spectral measurement of supersonically expanded intermediates on a cold substrate or by the technique of argon-matrix isolation as well as computationally at harmonic and one-dimensional anharmonic levels. The computations were based on DFT-D, MP2, MP2-F12, and CCSD(T)-F12 levels using various extended AO basis sets. The formation of all complexes related to the redshift of the Si−H stretching frequency upon complex formation was accompanied by an increase in its intensity. Similar results were obtained for another 10 electron acceptors of different types, positive σ-, π-, and p-holes and cations. The formation of HBe−H···Y complexes, studied only computationally and again containing a hydridic hydrogen, was characterized by the blueshift of the Be−H stretching frequency upon complexation accompanied by an increase in its intensity. The spectral shifts and stabilization energies obtained for all presently studied hydridic H-bonded complexes were comparable to those in protonic H-bonded complexes, which has prompted us to propose a modification of the existing IUPAC definition of H-bonding that covers, besides the classical protonic form, the non-classical hydridic and dihydrogen forms.Web of Science145158559855

S66: A Well-balanced Database of Benchmark Interaction Energies Relevant to Biomolecular Structures

With numerous new quantum chemistry methods being developed in recent years and the promise of even more new methods to be developed in the near future, it is clearly critical that highly accurate, well-balanced, reference data for many different atomic and molecular properties be available for the parametrization and validation of these methods. One area of research that is of particular importance in many areas of chemistry, biology, and material science is the study of noncovalent interactions. Because these interactions are often strongly influenced by correlation effects, it is necessary to use computationally expensive high-order wave function methods to describe them accurately. Here, we present a large new database of interaction energies calculated using an accurate CCSD(T)/CBS scheme. Data are presented for 66 molecular complexes, at their reference equilibrium geometries and at 8 points systematically exploring their dissociation curves; in total, the database contains 594 points: 66 at equilibrium geometries, and 528 in dissociation curves. The data set is designed to cover the most common types of noncovalent interactions in biomolecules, while keeping a balanced representation of dispersion and electrostatic contributions. The data set is therefore well suited for testing and development of methods applicable to bioorganic systems. In addition to the benchmark CCSD(T) results, we also provide decompositions of the interaction energies by means of DFT-SAPT calculations. The data set was used to test several correlated QM methods, including those parametrized specifically for noncovalent interactions. Among these, the SCS-MI-CCSD method outperforms all other tested methods, with a root-mean-square error of 0.08 kcal/mol for the S66 data set

On the Different Characteristics of Medium-Sized van der Waals Molecules Obtainable from ab initio Calculations

Nonempirical ab initio calculations with inclusion of the correlation energy are used to generate, directly or indirectly (via an analytical form of the potential-energy surface), various properties of medium-sized van der Waals molecules. As shown for benzene- • -Ar and the benzene dimer, the calculated characteristics agree with the experimental results. If the experimental values are lacking or if they are uncertain, the theoretical characteristics can be used with confidence