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The effectiveness of the optical rotation prediction (ORP) basis set for computing specific rotations at the coupled cluster (CC) level has been evaluated for a test set of 14 chiral compounds. For this purpose, the ORP basis set has been developed for the second-row atoms present in the investigated systems (that is, for sulfur, phosphorus, and chlorine). The quality of the resulting set was preliminarily evaluated for seven molecules using time-dependent density-functional theory (TD-DFT). Rotations were calculated with the coupled cluster singles and doubles method (CCSD) as well as the second-order approximate coupled cluster singles and doubles method (CC2) with the correlation-consistent aug-cc-pVDZ and aug-cc-pVTZ basis sets and extrapolated to estimate the complete basis-set (CBS) limit for comparison with the ORP basis set. In the compounds examined here, the ORP calculations on molecules containing only first-row atoms compare favorably with results from the larger aug-cc-pVTZ basis set, in some cases lying closer to the estimated CBS limit, while results for molecules containing second-row atoms indicate that larger correlation-consistent basis sets are necessary to obtain reliable estimates of the CBS limit.

Even though (H2O)2 and (HF)2 are arguably the most thoroughly characterized prototypes for hydrogen bonding, their heterogeneous analogue H2O···HF has received relatively little attention. Here we report that the experimental dissociation energy (D0) of this important paradigm for heterogeneous hydrogen bonding is too large by 2 kcal mol–1 or 30% relative to our computed value of 6.3 kcal mol–1. For reference, computational procedures similar to those employed here to compute D0 (large basis set CCSD(T) computations with anharmonic corrections from second-order vibrational perturbation theory) provide results within 0.1 kcal mol–1 of the experimental values for (H2O)2 and (HF)2. Near the CCSD(T) complete basis set limit, the electronic dissociation energy for H2O···HF is ∼4 kcal mol–1 larger than those for (H2O)2 and (HF)2 (∼9 kcal mol–1 for the heterogeneous dimer vs ∼5 kcal mol–1 for the homogeneous dimers). Results reported here from symmetry-adapted perturbation theory computations suggest that this l...

The calculation of specific rotation of molecules in solution is probed at the coupled cluster (CC) level utilizing a continuum dielectric model based on a definition of the dielectric permittivity as a smooth function of electron density. Solvation effects are captured through polarization of Hartree-Fock (HF) molecular orbitals before subsequent calculations with the coupled cluster singles and doubles (CCSD) method. For the challenging ( S)-methyloxirane molecule, CCSD specific rotations yield an incorrect sign for the rotation in water, and the continuum model is unable to predict the wide variations in the optical rotatory dispersion (ORD) curves seen for nonpolar solvents of similar dielectric constant. In two molecules, (1 S,4 S)-norbornenone and ( S)-2-chloropropionitrile, specific rotations computed with CCSD in conjunction with implicit solvent fail to provide solvent shifts of the correct order of magnitude, indicating that the solvent response is a major contribution to the overall solvation effect.

Even though (H

A series of (H2O)n clusters ranging from the dimer to the hexamer have been characterized with the CCSD(T) and the 2-body:Many-body CCSD(T):MP2 methods near the complete basis set (CBS) limit to generate benchmark-quality optimized structures and harmonic vibrational frequencies for these important systems. Quadruple-ζ correlation-consistent basis sets that augment the O atoms with diffuse functions have been employed in the analytic computation of harmonic vibrational frequencies for the global minima of the dimer, trimer, tetramer, and pentamer as well as the ring, book, cage, and prism isomers of the hexamer. Prior calibration [J. Chem. Phys. 2013, 139, 184113 and J. Chem. Theory Comput. 2014, 10, 5426] suggests that harmonic frequencies computed with this approach will lie within a few cm(-1) of the canonical CCSD(T) CBS limit. These data are used as reference values to gauge the performance of harmonic frequencies obtained with other ab initio methods (e.g., LCCSD(T) and MP2) and water potentials (e.g., TTM3-F and WHBB). This comparison reveals that it is far more challenging to converge harmonic vibrational frequencies for the bound OH stretching modes in these (H2O)n clusters to the CCSD(T) CBS limit than the free OH stretches, the n intramonomer HOH bending modes and even the 6n - 6 intermonomer modes. Deviations associated with the bound OH stretching harmonic frequencies increase rapidly with the size of the cluster for all methods and potentials examined, as do the corresponding frequency shifts relative to the monomer OH stretches.

We present a method for computing excitation energies for molecules in solvent, based on the combination of a minimal parameter implicit solvent model and the equation-of-motion coupled-cluster singles and doubles method (EOM-CCSD). In this method, the solvent medium is represented by a smoothly varying dielectric function, constructed directly from the quantum mechanical electronic density using only two tunable parameters. The solvent–solute electrostatic interactions are computed by numerical solution of the nonhomogeneous Poisson equation and incorporated at the Hartree–Fock stage of the EOM-CCSD calculation by modification of the electrostatic potential. We demonstrate the method by computing excited state transition energies and solvent shifts for several small molecules in water. Results are presented for solvated H2O, formaldehyde, acetone, and trans-acrolein, which have low-lying n → π* transitions and associated blue shifts in aqueous solution. Comparisons are made with experimental data and other theoretical approaches, including popular implicit solvation models and QM/MM methods. We find that our approach provides surprisingly good agreement with both experiment and the other models, despite its comparative simplicity. This approach only requires modification of the Fock operator and total energy expressions at the Hartree–Fock level—solvation effects enter into the EOM-CCSD calculation only through the Hartree–Fock orbitals. Our model provides a theoretically and computationally simple route for accurate simulations of excited state spectra of molecules in solution, paving the way for studies of larger and more complex molecules.

This collection of case studies, based on factual situations which have challenged broadcast managers in recent years, is designed to stimulate thinking about and solving of "real world" problems in commercial radio and television operations. Topics of a serious, long-run nature include enlarging the radio audience; station revenue and economy; programming, sales, and promotion in new ultra-high frequency (UHF) stations; the late evening news-weather-sports format; being independent in a four-station market; and filling a top station management position. Other subjects, which are more short term and don't involve capital investment, concern potential lawsuits, the controversy over broadcast editorializing, public service and cultural programming, and in-fighting among local media. Each problem is written as a dialogue among station personnel, and after the situation has been established, the reader is presented with a decision to make about the problem. Each chapter lists additional readings which bear on the problem, and in class situations, problems may be worked through using role-playing techniques. (JK)

Although the first ab initio Hartree–Fock computations of the water dimer were reported more than four decades ago, the detailed characterization of water clusters with sophisticated electronic structure techniques remains an important and vibrant area of research. The field of computational quantum chemistry has made significant advances since those pioneering studies. Geometry optimizations of the water dimer can now be carried out at the CCSDTQ level, and CCSD(T) energies can be computed with the aug-cc-pVTZ basis for clusters as large as (H2O)17. Some of these high-level studies are starting to reveal that the electronic structure is harder to describe for some hydrogen bonds than others. For example, discrepancies between MP2 and CCSD(T) energetics tend to increase when there are qualitative differences in the hydrogen-bonding networks of the water clusters being studied. This review highlights the recent and exciting work in this area and provides an overview of popular strategies for generating reliable properties and benchmark quality energetics for water clusters with correlated wavefunction methods.

More than 70 unique micro-hydrated structures of pyrimidine, ranging in size from 1 to 7 water molecules, have been characterized with the B3LYP density functional and the 6-311++G(2df,2pd) triple-ζ split-valence basis set. Explicitly correlated MP2-F12 single-point computations were performed on each structure with a correlation consistent triple-ζ basis set to estimate the relative and dissociation energies at the MP2 complete basis set (CBS) limit. Many of these new structures have significantly lower energies than those previously reported (by as much as 12.66 kcal mol(-1)). For clusters with 1 and 2 water molecules, the MP2-F12 relative and dissociation energies are virtually identical to the corresponding CCSD(T)-F12 values. As the number of hydrating waters increases, the structures in which the water molecules are clustered together at one of the N atoms have lower energies than those where the water molecules are more distributed around the pyrimidine ring. Micro-hydrated structures that effectively extend the low-energy hydrogen-bonding motifs to both sides of the ring, as would be expected in the bulk phase, reproduce the experimentally observed vibrational frequency shifts of ν(1) and ν(8b) in very dilute aqueous pyrimidine solutions to within 1 cm(-1) . Micro-hydrated structures of pyrimidine in which water molecules are clustered together have lower energies than structures in which the water molecules are more evenly spread around the pyrimidine ring.

This work presents a systematic investigation into the basis set convergence of harmonic vibrational frequencies of (H2O)2 and (HF)2 computed with second-order Moller-Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles method with perturbative connected triples, CCSD(T), while employing correlation-consistent basis sets as large as aug-cc-pV6Z. The harmonic vibrational frequencies presented here are expected to lie within a few cm(-1) of the complete basis set (CBS) limit. For these important hydrogen-bonding prototype systems, a basis set of at least quadruple-ζ quality augmented with diffuse functions is required to obtain harmonic vibrational frequencies within 10 cm(-1) of the CBS limit. In addition, second-order vibrational perturbation theory (VPT2) anharmonic corrections yield CCSD(T) vibrational frequencies in excellent agreement with experimental spectra, differing by no more than a few cm(-1) for the intramonomer fundamental vibrations. D0 values predicted by CCSD(T) VPT2 computations with a quadruple-ζ basis set reproduce the experimental values of (HF)2 and (H2O)2 to within 2 and 21 cm(-1), respectively.

We present an efficient method for reproducing CCSD(T) (i.e., the coupled-cluster method with single, double and perturbative connected triple excitations) optimized geometries and harmonic vibrational frequencies for molecular clusters with the N-body:Many-body QM:QM technique. In this work, all 1-body through N-body interactions are obtained from CCSD(T) computations, and the higher-order interactions are captured at the MP2 level. The linear expressions from the many-body expansion facilitate a straightforward evaluation of geometrical derivative properties (e.g., gradients and Hessians). For (H2O)n clusters (n = 3-7), optimized structures obtained with the 2-body:Many-body CCSD(T):MP2 method are virtually identical to CCSD(T) optimized geometries. Harmonic vibrational frequencies calculated with this 2-body:Many-body approach differ from CCSD(T) frequencies by at most a few cm(-1). These deviations can be systematically reduced by including more terms from the many-body expansion at the CCSD(T) level. Maximum deviations between CCSD(T) and 3-body:Many-body CCSD(T):MP2 frequencies are typically only a few tenths of a cm(-1) for the H2O clusters examined in this work. These results are obtained at a fraction of the wall time of the supermolecular CCSD(T) computation, and the approach is well-suited for parallelization on relatively modest computational hardware.

A comprehensive Raman spectroscopic/electronic structure study of hydrogen bonding by pyrimidine with eight different polar solvents is presented. Raman spectra of binary mixtures of pyrimidine with methanol and ethylene glycol are reported, and shifts in ν1, ν3, ν6a, ν6b, ν8a, ν8b, ν9a, ν15, ν16a, and ν16b are compared to earlier results obtained for water. Large shifts to higher vibrational energy, often referred to as blue shifts, are observed for ν1, ν6b, and ν8b (by as much as 14 cm(-1)). While gradual blue shifts with increasing hydrogen bond donor concentration are observed for ν6b and ν8b, ν1 exhibits three distinct spectral components whose relative intensities vary with concentration. The blue shift of ν1 is further examined in binary mixtures of pyrimidine with acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Electronic structure computations for more than 100 microsolvated structures reveal a significant dependence of the magnitude of the ν1 blue shift on the local microsolvation geometry. Results from natural bond orbital (NBO) calculations also reveal a strong correlation between charge transfer and blue shifting of pyrimidine's normal modes. Although charge transfer has previously been linked to blue shifting of the X-H stretching frequency in hydrogen bond donors, here, a similar trend in a hydrogen bond acceptor is demonstrated.

A wide range of density functional theory (DFT) methods (37 altogether), including pure, hybrid, range-separated hybrid, double-hybrid, and dispersion-corrected functionals, have been employed to compute the harmonic vibrational frequencies of eight small water clusters ranging in size from the dimer to four different isomers of the hexamer. These computed harmonic frequencies have been carefully compared to recently published benchmark values that are expected to be very close to the CCSD(T) complete basis set limit. Of the DFT methods examined here, ωB97 and ωB97X are the most consistently accurate, deviating from the reference values by less than 20 cm(-1) on average and never more than 60 cm(-1). The performance of double-hybrid methods including B2PLYP and mPW2-PLYP is only slightly better than more economical approaches, such as the M06-L pure functional and the M06-2X hybrid functional. Additionally, dispersion corrections offer very little improvement in computed frequencies.