High-level CCSD Research Articles

Matrix Isolation and Computational Study of the [H, C, N, Se] Isomers

Nine minima on the ground-state singlet and ten minima on the lowest-energy triplet potential energy surfaces of the [H, C, N, Se] system were located at the B3LYP/aug-cc-pVTZ level of theory. The singlet isomers were further investigated by the higher-level CCSD(T) method. Besides their structure and relative energies, isomerization barriers and the dissociation energies of the most important fragmentation channels were determined. Anharmonic vibrational wavenumbers, infrared intensities, relative Raman intensities, and UV excitation energies were also computed to assist the detection of these species. Two of the singlet isomers were generated and investigated by IR and UV spectroscopic methods. First, HNCSe and its deuterated isotopomer, DNCSe, were prepared by the reaction of HBr/DBr with AgNCSe and deposited in an 8 K Ar matrix. Photolysis of (H/D)NCSe at 254 nm led to the formation of the novel (H/D)SeNC isomer, which decomposed upon broad-band UV irradiation.

The OH-initiated atmospheric oxidation of cyclopentene: A coupled-cluster study of the potential energy surface

We performed the first theoretical potential energy surface investigation on the mechanism and products of the reaction of OH+ cyclopentene in the absence and presence of O2 by using high-level quantum chemical methods CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE×0.9335. Energies for several species are also refined at the CCSD(T)/cc-pVTZ levels of theory. The calculations indicate that the major products are cyclopentanone, 1-cyclopenten-1-ol, and 2-cyclopenten-1-ol in the absence of O2, which are in qualitative accordance with the available experimental observations. In the presence of O2, the dominant products are predicted to be glutaraldehyde and 1,2-epoxycyclopentanol.

A Theoretical Study of the OH + HN<sub>2</sub> Reaction

The singlet potential surface of the OH + HN2 reaction has been investigated at the B3LYP/6-311++G (3df, 3pd) level. The single-point energy calculations are performed at the high-level CCSD (T) / 6-311++G (3df, 3pd) for more accurate energy values. DFT calculations show that the most favorable association way starts with a barrierless addition of the OH to HN2 leading to HNN-OH (im6). The adduct im6 goes through an H shift from N atom to O atom, forming the product of H2O and N2.

Computational Studies of Bridging Structures and Isomerism in Substituted Disilynes

The substituted disilyne molecules, Si2Li2 and Si2HX, where X = Li, F, and Cl, have been investigated using the high-level CCSD(T) and CCSD(T)-F12 ab initio methods. The calculations have found or confirmed the existence of several isomeric forms and transition states for each molecule. Optimized geometries, relative energies, and harmonic vibration frequencies are reported. Bridging structures exist in all cases. Comparisons are made with existing literature results for the related Si2H2, C2X2, and C2HX isomerizing systems. Additionally, CCSD(T) and CCSD(T)-F12 calculations were performed for Si2H2, for which experimental spectroscopic data are available. Results calculated with CCSD(T)-F12 and the cc-pVTZ-F12 basis set are of comparable quality as those computed with CCSD(T) and the much larger cc-pV(6+d)Z basis set, at much less computational cost. We recommend the CCSD(T)-F12/cc-pVTZ-F12 level of theory as a very attractive alternative to conventional CCSD(T).

Microwave Detection of Sulfoxylic Acid (HOSOH)

Sulfoxylic acid (HOSOH), a chemical intermediate roughly midway along the path between highly reduced (H2S) and highly oxidized sulfur (H2SO4), has been detected using Fourier transform microwave spectroscopy and double resonance techniques, guided by new high-level CCSD(T) quantum-chemical calculations of its molecular structure. Rotational spectra of the two most stable isomers of HOSOH, the putative ground state with C2 symmetry and the low-lying C(s) rotamer, have been measured to high precision up to 71 GHz, allowing accurate spectroscopic parameters to be derived for both isomers. HOSOH may play a role in atmospheric and interstellar chemistry, and the present work provides the essential data to enable remote sensing and/or radioastronomical searches for these species. Spectroscopic characterization of HOSOH suggests that other transient intermediates in the oxidation of SO2 to H2SO4 may be amenable to laboratory detection as well.

Accurate Computation of Cohesive Energies for Small to Medium-Sized Gold Clusters.

High-level CCSD(T)-F12-type procedures have been used to assess the performance of a variety of computationally less demanding methods for the calculation of cohesive energies for small to medium-sized gold clusters. For geometry optimization for small gold clusters, the PBE-PBE/cc-pVDZ-PP procedure gives structures that are in close agreement with the benchmark geometries. We have devised a CCSD(T)-F12b-based composite protocol for the accurate calculation of cohesive energies for medium-sized gold clusters. Using these benchmark (nonspin-orbit vibrationless) cohesive energies, we find that fairly good agreement is achieved by the PBE-PBE-D3/cc-pVTZ-PP method. In conjunction with PBE-PBE/cc-pVDZ-PP zero-point vibrational energies and spin-obit corrections obtained with the PBE-PBE-2c/dhf-TZVP-2c method, we have calculated 0 K cohesive energies for Au2-Au20. Extrapolation of these cohesive energies to bulk yields an estimated value of 383.2 kJ mol(-1), which compares reasonably well with the experimental value of 368 kJ mol(-1).

Atmospheric Reactivity of CH2ICl with OH Radicals: High-Level OVOS CCSD(T) Calculations for the X-Abstraction Pathways (X = H, Cl, or I)

Reactants, weak molecular complexes, transition states, and products for the H-, Cl-, and I-abstraction channels in the reaction of OH radicals with chloroiodomethane CH(2)ICl as well as the energy profiles at 0 K have been determined using high-level all-electron ab initio methods. The results showed that all-electron DK-CCSD(T)/ANO-RCC approach performed very well in predicting the reactivity of iodine. In terms of activation enthalpy at 0 K, the energy profile for the Cl-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H- and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (-87 kJ mol(-1)), while the I-abstraction was modestly endothermic (11.8 kJ mol(-1)). On the basis of our calculations including the following corrections to the potential energies: basis set saturation, valence and core-valence electron correlation, relativistic effects, spin-adaptation, vibration contributions, and tunneling corrections, rate constants were predicted using canonical transition state theory over the temperature range 250-500 K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 4.29 × 10(-14) and 5.44 × 10(-14) cm(3) molecule(-1) s(-1) for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH(2)ICl. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

Mechanistic studies of ammonia borane dehydrogenation

Ammonia borane (NH3BH3, AB) has received extensive attention as a potential hydrogen storage medium, however hydrogen release mechanisms from AB are not well understood. AB follows different reaction routes if the dehydrogenation occurs in solvent or solid state, but a comparative study for AB dehydrogenation in these two states is not available. In this work, a detailed study of AB dehydrogenation mechanism in diglyme and solid state is presented, and a comprehensive reaction network for both cases is proposed. The experimental and DFT results suggest that two main reaction pathways occur; one involves cyclization of monomers which results in faster dehydrogenation at lower temperature, while the other involves propagation to acyclic intermediates which requires higher temperature to carry out the cyclization step. AB dehydrogenation in solid state was experimentally found to be initiated by B–N bond cleavage and not by direct dehydrogenation, which agrees with high level CCSD(T)/MP2 calculations reported previously. It was found that diglyme plays a significant role in hindering B–N bond cleavage of AB which facilitates the cyclization pathway. In solid state, experiments including labeled AB (ND3BH3) mapped out the source of hydrogen (from hydridic or protonic ends), and a clear difference in the degree of dehydrogenation from the two ends is demonstrated.

Appropriate description of intermolecular interactions in the methane hydrates: An assessment of DFT methods

Accurate description of hydrogen-bonding energies between water molecules and van der Waals interactions between guest molecules and host water cages is crucial for study of methane hydrates (MHs). Using high-level ab initio MP2 and CCSD(T) results as the reference, we carefully assessed the performance of a variety of exchange-correlation functionals and various basis sets in describing the noncovalent interactions in MH. The functionals under investigation include the conventional GGA, meta-GGA, and hybrid functionals (PBE, PW91, TPSS, TPSSh, B3LYP, and X3LYP), long-range corrected functionals (ωB97X, ωB97, LC-ωPBE, CAM-B3LYP, and LC-TPSS), the newly developed Minnesota class functionals (M06-L, M06-HF, M06, and M06-2X), and the dispersion-corrected density functional theory (DFT) (DFT-D) methods (B97-D, ωB97X-D, PBE-TS, PBE-Grimme, and PW91-OBS). We found that the conventional functionals are not suitable for MH, notably, the widely used B3LYP functional even predicts repulsive interaction between CH(4) and (H(2)O)(6) cluster. M06-2X is the best among the M06-Class functionals. The ωB97X-D outperforms the other DFT-D methods and is recommended for accurate first-principles calculations of MH. B97-D is also acceptable as a compromise of computational cost and precision. Considering both accuracy and efficiency, B97-D, ωB97X-D, and M06-2X functional with 6-311++G(2d,2p) basis set without basis set superposition error (BSSE) correction are recommended. Though a fairly large basis set (e.g., aug-cc-pVTZ) and BSSE correction are necessary for a reliable MP2 calculation, DFT methods are less sensitive to the basis set and BSSE correction if the basis set is sufficient (e.g., 6-311++G(2d,2p)). These assessments provide useful guidance for choosing appropriate methodology of first-principles simulation of MH and related systems. © 2012 Wiley Periodicals, Inc.

Ab initio prediction of vibrational states of the HeCuF helium-containing complex

A global three-dimensional potential energy surface for the HeCuF system, which has a relatively large helium binding energy of ca. 6.2kcal/mol, has been developed using the high-level ab initio CCSD(T) electronic structure method with scalar relativistic effects taken into account. Using the developed potential energy surface, vibrational states of 4HeCuF and 3HeCuF have been calculated quantum mechanically. Spectroscopic detection of these helium-containing complexes has been discussed.

Bonds or not bonds? Pancake bonding in 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl radical dimers and their derivatives

Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.

What is the best density functional to describe water clusters: evaluation of widely used density functionals with various basis sets for (H2O) n (n = 1–10)

The geometric structures, stabilization energies, dipole moments, and vibrational frequencies of the neutral water clusters (H2O)n, with n = 1–10, were investigated using density functional theory along with a variety of exchange-correlation functionals (LDA with SVWN5 parameterization, GGA with BLYP, PW91, PBE, B3LYP, X3LYP, PBE0, PBE1W, M05-2X, M06-2X and M06-L parameterizations) as well as high-level ab initio MP2 and CCSD(T) methods. Using the MP2 and CCSD(T) results as benchmarks, the effects of exchange-correlation functionals and basis sets were carefully examined. Each functional has its advantage in certain aspects; for example, M05-2X and X3LYP yield better geometries, and the capability of these two functionals to distinguish the relative energies between isomers are more similar to MP2. The size of the split-valence basis set (6-31G or larger), diffuse functions on the oxygen atom, and d(p) polarization on the oxygen (hydrogen) atom are crucial for an accurate description of intermolecular interaction in water clusters. The 6-31+G(2d,p) basis set is thus recommended as a compromise between computational efficiency and accuracy for structural description. We further demonstrated that the numerical basis set, TNP, performs satisfactorily in describing structural parameters of water clusters.

Quantum chemical studies on the role of water microsolvation in interactions between group 12 metal species (Hg2+, Cd2+, and Zn2+) and neutral and deprotonated cysteines

Interactions of group 12 metal(II) species (Hg2+, Cd2+, Zn2+, Hg(H2O) 2+ , Cd(H2O) 2+ , and Zn(H2O) 2+ (n = 1, 2) with neutral (RSH), deprotonated (RS−), and doubly deprotonated cysteine species (abbreviated as “H2cys”, “Hcys−”, and “cys2−”, respectively) are examined with the Becke three-parameter Lee–Yang–Parr (B3LYP) hybrid functional after preliminary screening in a conformation analysis with the Parameterized Model number 3 (PM3) semiempirical method. Effects of water on aqueous solution are evaluated by microsolvation and polarized continuum model (PCM) approaches. In the most stable conformations of M(H2cys)2+ and M(Hcys)+ complexes (M = Hg2+, Cd2+, and Zn2+), the SH group of the cysteine moiety is already deprotonated and undergoes strong binding with the metal ion. Among Hg(H2cys)2+ complexes, cysteine complexes of Hg2+ without deprotonation of the SH group and mercury(II) carboxylato-type structures are at least 83 and 117 kJ/mol less stable in energy than the most stable complex (B3LYP/6-311++G(d,p)-SDD+d+f//B3LYP/6-31G(d)-SDD+d). Although Zn2+ binds more strongly than Hg2+ to a H2cys molecule at the high-level CCSD(T)/6-311++G(d,p)-SDD+d+f//B3LYP/6-311++G(d,p)-SDD+d+f level, [Hg(H2O)2]2+ is stronger than [Zn(H2O)2]2+ because the deformation of [Zn(H2O)2]2+ required to bind to cys is much more than in [Hg(H2O)2]2+. Complexes with a deprotonated cysteine, M(Hcys)+ and M(cys), prefer a multidentate structure.

Bond Energies in Models of the Schrock Metathesis Catalyst

Heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) of the model Schrock-type metal complexes M(NH)(CRR′)(OH)2 (M = Cr, Mo, W; CRR′ = CH2, CHF, CF2) and MO2(OH)2 compounds, and Brønsted acidities and fluoride affinities for the M(NH)(CH2)(OH)2 transition metal complexes are predicted using high level CCSD(T) calculations. The metallacycle intermediates formed by reaction of C2H4 with M(NH)(CH2)(OH)2 and MO2(OH)2 are investigated at the same level of theory. Additional corrections were added to the complete basis set limit to obtain near chemical accuracy (±1 kcal/mol). A comparison between adiabatic and diabatic BDEs is made and provides an explanation of trends in the BDEs. Electronegative groups bonded on the carbenic carbon lead to less stable Schrock-type complexes as the adiabatic BDEs of M═CF2 and M═CHF bonds are much lower than the M═CH2 bonds. The Cr compounds have smaller BDEs than the W or Mo complexes and should be less stable. Different M(NH)(OH)2(C3H6) and MO(OH)2(OC2H4) metallacycle intermediates are investigated, and the lowest-energy metallacycles have a square pyramidal geometry. The results show that consideration of the singlet–triplet splitting in the carbene in the initial catalyst as well as in the metal product formed by the retro [2 + 2] cycloaddition is a critical component in the design of an effective olefin metathesis catalyst in terms of the parent catalyst and the groups being transferred.

NXAl3+ (X = N, P, As): penta-atomic planar tetracoordinate nitrogen with N–X multiple bonding

Planar tetracoordinate nitrogen (ptN) has been successfully extended as a new branch of planar chemistry. As the simplest type of ptN, penta-atomic species (pptN, e.g., NAl(4)(-)) are known to have a "delocalized" molecular environment with a single bond between the central N and the ligand. In this paper, through an extensive isomeric search of a series of group V-based systems NXAl(3)(+) (X = N, P and As) in both singlet and triplet electronic states at the B3LYP/6-311+G(d) level, we report a class of novel pptN with unique chemical bonding, i.e., the central nitrogen and the connected ligand X (X = N, P and As) effectively form a highly "localized" N-X multiple bond, as confirmed by the aug-cc-pVTZ-B3LYP and MP2 calculations. The high-level CCSD(T)/aug-cc-pVTZ energetic calculations show that the three pptN species each have appreciable kinetic stability against structural transformation and fragmentation, which is confirmed by the Born-Oppenheimer molecular dynamics calculations. Possible formation pathways of the three pptNs are discussed. In particular, the pptN isomer with X = P, i.e., NPAl(3)(+), is the global minimum, making the pptN-based NPAl(3)(+) the most accessible via mass spectroscopic characterization. The present work demonstrates that the frequently used concept "localization vs. delocalization" in organic chemistry can also be transplanted to exotic planar chemistries like pptN.

Electronic structure and lifetimes of GaX2+ (X = N, O, F) in the gas phase. Unraveling stability trends

High-level single-reference CCSD(T) and multireference MS-CASPT2/CASSCF ab initio calculations have been carried out to determine the electronic structure and the lifetimes of GaX(2+) (X = N, F) doubly charged diatomic systems. Lifetimes were evaluated using the Exterior Complex Scaling (ECS) method implemented with basis sets of B-spline functions. The results obtained led to the conclusion that both species GaN(2+) and GaF(2+) can be considered as bound systems in the gas phase. GaN(2+) is a kinetically stable dication, because although it is thermodynamically unstable with respect to its dissociation into Ga(+) ((1)S) + N(+) ((3)P) the barrier to be surmounted is rather high and wide, so that the lowest 14 vibrational states of this system are bound. GaF(2+) is found to be a thermodynamically stable species with respect to its dissociation into Ga(+) ((1)S) + F(+) ((1)D). With respect to its dissociation into Ga(+) ((1)S) + F(+) ((3)P), strictly speaking, it is metastable. However, the crossing between the attractive PEC and the repulsive one occurs at rather large internuclear distances, and therefore it can be only explored by highly vibrationally excited molecules. Thus, in practical terms GaF(2+) can be considered as a bound species. The behavior of GaN(2+) and GaF(2+) is in clear contrast with that predicted for GaO(2+), which was found to be metastable with respect to its dissociation into Ga(+) ((1)S) + O(+) ((4)S), with lifetimes in the range of ns to seconds. This seems to indicate that the primary factor governing the stability and lifetimes of these doubly charged diatomic species is the ionization energy of the atom X bound to Ga.

Structural and electronic trends among group 15 polyhedral fullerenes

We have investigated the structural and electronic characteristics of tetrahedral, octahedral, and icosahedral fullerenes composed of group 15 elements phosphorus, arsenic, antimony, and bismuth. Systematic quantum chemical studies at the DFT and MP2 levels of theory were performed to obtain periodic trends for the structural principles, stabilities, and electronic properties of the elemental nanostructures. Calibration calculations for polyhedral clusters with up to 20 atoms showed the applied theoretical approaches to be in good agreement with high-level CCSD(T)/cc-pVTZ results. By studying fullerenes up to P888, As540, Sb620, and Bi620, we found their structures and stabilities to converge smoothly toward their experimental bulk counterparts. The diameters of the largest studied cages were 4.8, 3.7, 4.8, and 5.1 nm for the P, As, Sb, and Bi fullerenes, respectively. Comparisons with the experimentally known allotropes of the studied elements suggest the predicted polyhedral cages to be thermodynamically stable. All studied group 15 polyhedral fullerenes were found to be semiconducting, and density of states analysis illustrated clear periodic trends in their electronic structure. Relativistic effects become increasingly important when moving from P to Bi and taking the spin–orbit effects into account by using a two-component procedure had a significant positive effect on the relative stability of bismuth clusters.

Bonding and Isomerism in SFn−1Cl (n = 1−6): A Quantum Chemical Study

Using high-level MRCI and CCSD(T) quantum chemical calculations, we report structures, energetics, and other properties of the sulfur fluoromonochloride family (SF(n-1)Cl, n = 1-6). Our group previously studied the sulfur fluoride family (SF(n), n = 1-6) and found that several of the excited states of SF and SF(2) as well as the ground states of SF(3)-SF(6) exhibited a new type of bonding, called recoupled pair bonding. Comparing the SF(n-1)Cl and SF(n) species allows us to study isomerism, apicophilicities, and substituent effects due to the Cl substitution. The primary findings of this work are twofold. First, replacing F with Cl weakens the adjacent S-F bonds by destabilizing the molecule with respect to the pure SF(n) analog. Second, an isomer with a singly occupied S-Cl antibonding orbital is more stable than the analogous isomer with a singly occupied S-F antibonding orbital, thus explaining apicophilicities. This work has also allowed us to further refine and expand our understanding of the nature of the recoupled pair bond model. Finally, we discovered the presence of bond-stretch isomers in the first excited ((3)A'') state of SFCl.

Direct dynamics study on mechanism and kinetics of the biradical self-reaction of HOO

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of the biradical hydroperoxy radical has been presented at the CCSD(T)/6-311++G(3d,2p)//CCSD/6-31+G(d,p) level of theory. Our theoretical calculations suppose a stepwise mechanism involving the formation of a postreactant complex in the triplet and singlet entrance channels. Four transition states of the six-membered chain complexes (3TS1 and 1TS1) and six-membered ring complexes (3TS2 and 1TS2) are located at the high dual level CCSD(T)/6-311++G(3d,2p)//CCSD/6-31+G(d,p) method. The rate constants of Path 1 ∼ Path 4 at the CCSD(T)/6-311++G(3d,2p)//CCSD/6-31+G (d,p) level are calculated by means of the conventional transition state theory (TST) and canonical variational TST without and with small-curvature tunneling (SCT) correction within the temperature range of 200–2,500 K. The calculated results show that the triplet channel is the dominating reaction channel and Path 2 is found to be the most favorable pathway. The rate constants of Path 2 are in good agreement with the experimental values at the experimentally measured temperatures. Moreover, the variational effect is not obvious in the low temperature range but is not neglectable in the high temperature range. The SCT plays an important role particularly in the low temperature range. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Simultaneous Two-Hydrogen Transfer as a Mechanism for Efficient CO2 Reduction

Two-hydrogen transfer (simultaneous protic and hydridic hydrogen transfer) is examined as a potentially efficient mechanism for the selective reduction of CO(2) to methanol. High-level ab initio CCSD(T) coupled-cluster theory simulations of ammonia-borane (AB), which contains both protic and hydridic hydrogen, show the effectiveness of this mechanism. AB demonstrates how simultaneous two-hydrogen transfer is kinetically efficient because (1) two-hydrogen transfer avoids high-energy single-electron-reduced intermediates, (2) the CO(2)'s HOMO is protonated while the LUMO is concurrently reduced by a hydride, and (3) complementary charge polarities around the six-membered-ring transition-state structures stabilize the transition states. This study suggests that an effective mechanism for the reduction of CO(2) to methanol proceeds through three two-hydrogen-transfer steps and that suitable catalysts should be developed that exploit two-hydrogen transfer without the use of AB.