Behind the success of modified coupled-cluster methods: addition by subtraction

ABSTRACT

Modified coupled-cluster (CC) methods such as linearized coupled-cluster doubles (LinCCD), approximate coupled pair (ACP D14), 2CC (from nCC family), parameterized CCSD (pCCSD) and distinguishable cluster (DCSD) can have their advantages over general CC methods. Though these methods include connected clusters of single and double excitations at most, distinguishable cluster, parameterized CC and approximate coupled pair methods, in particular, have been shown to produce quantitatively correct results in benchmark studies. To put these methods on a stronger foothold, it is essential to understand the rationale for their success: mimicking the effect of connected triple excitations. We exploit the relation between CC and many body perturbation theory (MBPT) in general, and between CCSD and MBPT(4)/MP4 in particular, to take a step towards bringing clarity to this persisting conundrum. Our aim here is to look for numerical signs of ‘addition by subtraction’ or ‘inclusion by deletion’ effect that is likely behind the success of these modified CCD or CCSD methods. We achieve this by revisiting well-studied examples of single and multiple bond dissociation and comparing the performance of these modified CCSD methods with higher-level CC methods. Though our results are qualitative in nature, we hope this would lead to more rigorous analysis in future studies.     

Could diastereoselectivity in the presence of O-2 chiral nonparticipating groups be an indicator of glycopyranosyl oxacarbenium ions in glycosylation reactions?

Although long postulated, the existence of glycopyranosyl oxacarbenium ions as intermediates or transition states (TS) in chemical glycosylation reactions has not been convincingly demonstrated experimentally. It is anticipated that elucidation of such reactive species will greatly assist synthetic chemists to control the α/β stereoselectivity by rational means. Previous density functional theory (DFT) calculations from our group found that the torsion potential about C-2-O-2 in protected glycopyranosyl donors changed from a conventional 3-fold rotor to a 2-fold rotor with a strong syn (CH-2-C-2-O-2-CPg) preference once the donor was ionized to its oxacarbenium ion. This suggested to us that if CPg of the protecting group was a chiral carbon, then diastereoselectivity might be observed in glycosylation reactions that proceed through oxacarbenium ions. The hypothesis to test is as follows: if a nonparticipating O-2 racemic chiral protecting group exhibits diastereoselectivity in glycosylation reactions, then the reaction probably proceeds through an oxacarbenium ion intermediate or TS. We present data for O-2 ether-protected d-glucopyranosyl donors where the racemic protecting group 1-methyl 1'-methylcyclopropylmethyl (MCPM) provides the chirality. MCPM proves to be more activating than the O-2-benzyl ether, and in cases where the donor is otherwise deactivated, several examples of moderate diastereoselectivity are found. These results can be interpreted to indicate that a continuum of reactivity exists where some glycosyl donors form oxacarbenium ions in glycosylation reactions but more reactive donors do not. The strongly activating properties of the cyclopropylmethyl ether functionality and the ability to induce diastereoselectivity with chiral derivatives strongly suggest widespread applications.     

Kinetics and mechanism of the oxidation of n-acetyl l-cysteine by 2,6-dichloroindophenol in methanol–water medium

The principal reactants interact in a molar ratio of 2:1 forming dihydroindophenol and the corresponding disulfide. The order of reaction is unity with respect to each reactant, namely, N-acetyl L-cysteine and indophenol. The rate shows an inverse linear relation with hydroxyl ions. It is not influenced by the variation in the initial concentration of indophenol. The rate increases on adding the salts possessing a cholinergic effect and the neutral electrolytes, as well as on increasing the dielectric constant of the medium. The products, namely, N-acetyl L-cystine (disulfide) and the leuco dye, do not influence the rate. A low energy and a highly negative entropy of activation (4.11 kcal/mol and -60.49 e.u., respectively) suggest the formation of an outer sphere complex as the rate-determining step.     

Origin of the high-and low-frequency K-satellites of 31Ga, 32Ge and 33As

The origin of the high- and low-frequency K-satellites of ³¹Ga, ³²Ge, ³³ As is explain on the basis of Hayasi's theory of quasi-stationary states. It is shown that the same QSS difference gives rise to a particular satellite of either type in all the elements. The results are compared with those of multiple ionisation theory.     

Forbidden hyperfine transitions in electron spin resonance of Mn2+ in NaCl single crystal

Forbidden hyperfine transitions are observed in the electron spin resonance spectrum of divalent Mn⁵⁵ ion in NaCl single crystal for a particular associated pair. From the measurements of the M = + 1/2 → ?1/2, Δm = ± 1 transitions the parametersQ′ and Q″ of the nuclear electric quadrupole part of the spin-Hamiltonian H_o = Q′ [I_z ² ? 1/3 I (I + 1)] + Q″ (I_x ²?I_y ²) are found to be + 1.70 × 10^?4 cm.^?1 and +0.16 × 10^?4 cm.^?1 respectively.     

Crystal field contribution to Mössbauer isomer shift

The crystal field hamiltonian H_c = B₂⁰Y₂⁰ + B₂²(Y₂² + Y₂^?2) commonly used in the study of electron-spin resonance is employed to obtain new contributions to the ⁵⁷Fe³⁺ isomer shift in the Mössbauer effect. A numerical estimate of the shift is also presented.     

Ferroelectric soft-mode contribution to zero-field splitting: Paramagnetic resonance investigation of phase transition in Mn2+-doped NaNO2

The contribution of the ferroelectric soft-mode to the zero-field splitting is calculated and separated from the thermal contribution arising from the splitting due to the electron-phonon interaction. The predicted effect is compared with the measured values in Mn²⁺-doped NaNO₂.     

Directed denaturation: room temperature and stoichiometric unfolding of cytochrome C by a metalloporphyrin dimer

Using circular dichroism, UV-vis, and trypsin proteolysis, we have shown how a metalloporphyrin dimer induces the unfolding of a protein, cytochrome c, under physiologically relevant conditions and accelerates its rate of proteolytic degradation.     

Viscous potential flow analysis of Kelvin-Helmholtz instability with mass transfer and vaporization

Viscous potential flow analysis of Kelvin-Helmholtz instability with heat and mass transfer has been studied. A dispersion relation has been obtained. Stability criterion is given by a critical value of relative velocity. It has been found that heat and mass transfer has destabilizing effect on relative velocity when lower fluid viscosity is low while it has stabilizing effect when lower fluid viscosity is high. Various graphs have been plotted for relative velocity and growth rate. In statically unstable situation viscosity has stabilizing effect while in statically stable situation it has destabilizing effect.