Electronic structure of the S₁ state in methylcobalamin: Insight from CASSCF/MC‐XQDPT2, EOM‐CCSD, and TD‐DFT calculations

Abstract

The methylcobalamin cofactor (MeCbl), which is one of the biologically active forms of vitamin B12, has been the subject of many spectroscopic and theoretical investigations. Traditionally, the lowest-energy part of the photoabsorption spectrum of MeCbl (the so-called α/β band) has been interpreted as an S0→S1 electronic transition dominated by π→π* excitations associated with the C=C stretching of the corrin ring. However, a more quantitative band-shape analysis of the α/β spectral region, along with circular dichroism (CD), magnetic CD, and resonance Raman data, has revealed the presence of a second electronic transition that involves the Co-C(Me) bond weakening. Conversely, the lowest-energy excitations based on transient absorption spectroscopy measurements have been interpreted as metal-to-ligand charge transfer (MLCT) transitions. To resolve the existing controversy about the interpretation of the S1 state of MeCbl, calculations have been performed using two independent ab initio wavefunction-based methods. These include the modified variant of the second-order multiconfigurational quasi-degenerate perturbation theory (MC-XQDPT2), using complete active space self-consistent field orbitals, and the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) approach using restricted Hartree-Fock orbitals. It is shown that both ab initio methods provide a consistent description of the S1 state as having an MLCT character. In addition, the performance of different types of functionals, including hybrid (B3LYP, MPW1PW91, TPSSh), generalized-gradient-approximation-type (GGA-type) (BP86, BLYP, MPWPW91), meta-GGA (TPSS), and range-separated (CAM-B3LYP, LC-BLYP) approaches, has been examined and the results of the corresponding time-dependent density functional theory calculations have been benchmarked against the MC-XQDPT2 and EOM-CCSD data. The hybrid functionals support the interpretation in which the S1 state represents a π→π* transition localized on corrin, while pure GGA, meta-GGA, and LC-BLYP functionals produce results consistent with the MLCT assignment.

Benchmarking the completely renormalised equation-of-motion coupled-cluster approaches for vertical excitation energies

“…Their argument is based on the ∼0.3-0.4 eV lowering when going from EOMCCSD to CC3. However, as can be seen in our tables and as shown in previous studies [64,[81][82][83], it is not unusual to observe errors of this magnitude in EOMCCSD calculations for singly excited states. Indeed, the REL values characterising the 2 1 A 1 (π → π * ) and 3 1 A 1 (π → π * ) states of cyclopentadiene, of about 1.153 and 1.055, respectively, when the MP2/6-31G * geometry is employed, show that these states are predominantly oneelectron transitions and that double excitations do not significantly contribute to their wave functions.…”

“…However, EOMCCSD is not robust enough to obtain a quantitative description of many of such states, especially when larger polyatomic species are examined (cf., e.g., [64,[81][82][83]; for a thorough evaluation of a number of EOMCC methods, including EOMCCSD, illustrating the same, see [17,19,20,22,24,25]). It also fails to describe states with significant double excitation contributions.…”

Excited electronic states and internal conversion in cyanocobalamin

“…The photoexcitation of non-alkyl cobalamins is characterized by formation of short-lived electronically excited states that recover to the ground state on a picosecond timescale with little to no photolysis quantum yield. Time dependent density functional theory (TD-DFT) studies by Kozlowski and coworkers have identified a triplet sigma antibonding state that was suggested to mediate the photolysis of alkyl cobalamins [28,[30][31][32][33][34]. Detailed comparison of MeCbl and ethylcobalamin (EtCbl) led to the suggestion that changes in the barrier for crossing from the S 1 state to the dissociative channel accounted for the distinctive wavelength dependence and photolysis quantum yield following excitation of MeCbl.…”

Theoretical investigations of photophysics and spectroscopic properties in B12 cofactors.

“…Whereas the ground-state structural properties were essentially insensitive with respect to the functional used, in contrast, the excited states were very sensitive to the choice of XC functional. Recent theoretical studies 140,161,163,271 on cobalamins support the use of gradient-corrected functionals, such as BP86, when studying excited states of these systems compared to, for instance, the use of hybrid functionals.…”

“…109 However, the validity of hybrid functionals with respect to electronic excitations of cobalamins have been questioned 140,161,163,271 and numerical evidence suggests that GGA-type functionals perform better. From these investigations, the S 1 for MeCbl was assigned as a MLCT excitation, whereas LMCT described the lowest excitation of CNCbl, indicating the importance of Co d orbitals in the electronic transitions.…”