5 research outputs found
Mechanism of Co–C Bond Photolysis in Methylcobalamin: Influence of Axial Base
A mechanism
of Co–C bond photolysis in the base-off form
of the methylcobalamin cofactor (MeCbl) and the influence of its axial
base on Co–C bond photodissociation has been investigated by
time-dependent density functional theory (TD-DFT). At low pH, the
MeCbl cofactor adopts the base-off form in which the axial nitrogenous
ligand is replaced by a water molecule. Ultrafast excited-state dynamics
and photolysis studies have revealed that a new channel for rapid
nonradiative decay in base-off MeCbl is opened, which competes with
bond dissociation. To explain these experimental findings, the corresponding
potential energy surface of the S<sub>1</sub> state was constructed
as a function of Co–C and Co–O bond distances, and the
manifold of low-lying triplets was plotted as a function of Co–C
bond length. In contrast to the base-on form of MeCbl in which two
possible photodissociation pathways were identified on the basis of
whether the Co–C bond (path A) or axial Co–N bond (path
B) elongates first, only path B is active in base-off MeCbl. Specifically,
path A is inactive because the energy barrier associated with direct
dissociation of the methyl ligand is higher than the barrier of intersection
between two different electronic states: a metal-to-ligand charge
transfer state (MLCT), and a ligand field state (LF) along the Co–O
coordinate of the S<sub>1</sub> PES. Path B initially involves displacement
of the water molecule, followed by the formation of an LF-type intermediate,
which possesses a very shallow energy minimum with respect to the
Co–C coordinate. This LF-type intermediate on path B may result
in either S<sub>1</sub>/S<sub>0</sub> internal conversion or singlet
radical pair generation. In addition, intersystem crossing (ISC) resulting
in generation of a triplet radical pair is also feasible
Mechanism of Co–C Bond Photolysis in the Base-On Form of Methylcobalamin
A mechanism
of Co–C bond photodissociation in the base-on
form of the methylcobalamin cofactor (MeCbl) has been investigated
employing time-dependent density functional theory (TD-DFT), in which
the key step involves singlet radical pair generation from the first
electronically excited state (S<sub>1</sub>). The corresponding potential
energy surface of the S<sub>1</sub> state was constructed as a function
of Co–C and Co–N<sub>axial</sub> bond distances, and
two possible photodissociation pathways were identified on the basis
of energetic grounds. These pathways are distinguished by whether
the Co–C bond (path A) or Co–N<sub>axial</sub> bond (path B) elongates first. Although the final intermediate of
both pathways is the same (namely a ligand field (LF) state responsible
for Co–C dissociation), the reaction coordinates associated
with paths A and B are different. The photolysis of MeCbl is wavelength-dependent,
and present TD-DFT analysis indicates that excitation in the visible
α/β band (520 nm) can be associated with path A, whereas
excitation in the near-UV region (400 nm) is associated with path
B. The possibility of intersystem crossing, and internal conversion
to the ground state along path B are also discussed. The mechanism
proposed in this study reconciles existing experimental data with
previous theoretical calculations addressing the possible involvement
of a repulsive triplet state
Positive and Negative Contributions in the Solvation Enthalpy due to Specific Interactions in Binary Mixtures of C1–C4 <i>n</i>‑Alkanols and Chloroform with Butan-2-one
In
the paper, results of calorimetric measurements, IR spectra, and calculated <i>ab initio</i> stabilization energies of dimers are reported
for binary systems butan-2-one + (methanol, ethanol, propan-1-ol,
butan-1-ol, and chloroform). Changes in the total enthalpy of specific
interactions due to dissolution of butan-2-one in the alcohols, calculated
using equations derived in previous works, are positive. That results
from the endothermic breaking of the OH···OH
bonds not completely compensated by the exothermic effects of formation
of the OH···OC ones. Moreover, the
concentration of nonbonded molecules of butan-2-one is significant
even in dilute solutions, as is evidenced by the shape of the CO
stretching vibrations band in the IR spectra. Apart from that, the
spectra do not confirm 1:2 complexes in spite of two lone electron
pairs in the carbonyl group of butan-2-one capable of forming the
hydrogen bonds. The changes in enthalpy of specific interactions are
negative for dilute solutions of alcohols and chloroform in butan-2-one
and of butan-2-one in chloroform, because no hydrogen bonds occur
in pure butan-2-one. The experimental results are positively correlated
with the enthalpies estimated from the <i>ab initio</i> energies
using a simple “chemical reaction” approach
The Cobalt–Methyl Bond Dissociation in Methylcobalamin: New Benchmark Analysis Based on Density Functional Theory and Completely Renormalized Coupled-Cluster Calculations
The Co–C<sub>Me</sub> bond dissociation in methylcobalamin
(MeCbl), modeled by the Im–[Co<sup>III</sup>corrin]–Me<sup>+</sup> system consisting of 58 atoms, is examined using the coupled-cluster
(CC), density-functional theory (DFT), complete-active-space self-consistent-field
(CASSCF), and CASSCF-based second-order perturbation theory (CASPT2)
approaches. The multilevel variant of the local cluster-in-molecule
framework, employing the completely renormalized (CR) CC method with
singles, doubles, and noniterative triples, termed CR-CC(2,3), to describe higher-order
electron correlation effects in the region where the Co–C<sub>Me</sub> bond breaking takes place, and the canonical CC approach
with singles and doubles (CCSD) to capture the remaining correlation
effects, abbreviated as CR-CC(2,3)/CCSD,
is used to obtain the
benchmark potential energy curve characterizing the Co–C<sub>Me</sub> dissociation in the MeCbl cofactor. The Co–C<sub>Me</sub> bond dissociation energy (BDE) resulting from the CR-CC(2,3)/CCSD calculations for the
Im–[Co<sup>III</sup>corrin]–Me<sup>+</sup> system using
the 6-31G* basis set, corrected for the zero-point energies (ZPEs)
and the effect of replacing the 6-31G* basis by 6-311++G**, is about
38 kcal/mol, in excellent agreement with the experimental values characterizing
MeCbl of 37 ± 3 and 36 ± 4 kcal/mol. Of all DFT functionals
examined, the best dissociation energies and the most accurate description
of the Co–C<sub>Me</sub> bond breaking in the Im–[Co<sup>III</sup>corrin]–Me<sup>+</sup> system are provided by B97-D
and BP86 corrected for dispersion using the D3 correction of Grimme
et al., which give 35 and 40 kcal/mol, respectively, when the 6-311++G**
basis set is employed and when the results are corrected for ZPEs
and basis set superposition error. None of the other DFT approaches
examined provide results that fall into the experimental range of
the Co–C<sub>Me</sub> dissociation energies in MeCbl of 32–40
kcal/mol. The hybrid DFT functionals with a substantial amount of
the Hartree–Fock (HF) exchange, such as B3LYP, considerably
underestimate the calculated dissociation energies, with the magnitude
of the error being proportional to the percentage of the HF exchange
in the functional. It is argued that the overstabilization of diradical
structures that emerge as the Co–C<sub>Me</sub> bond is broken
and, to some extent, the neglect of dispersion interactions at shorter
Co–C<sub>Me</sub> distances, postulated in previous studies,
are the main factors that explain the substantial underestimation
of the Co–C<sub>Me</sub> BDE by B3LYP and other hybrid functionals.
Our calculations suggest that CASSCF and CASPT2 may have difficulties
with providing a reliable description of the Co–C<sub>Me</sub> bond breaking in MeCbl, since using adequate active spaces is prohibitively
expensive