MP2 calculations with the full aug-cc-pVTZ basis set give a non-planar structure for benzene. Although this non-physical result can be avoided by using the smaller aug-cc-pVDZ basis set or by scaling or deleting selected functions from the aug-cc-pVTZ basis set, such changes to the basis set can still result in calculated values of the frequencies of the b 2g out-of-plane vibrations that are considerably underestimated. The origin of this behavior is traced to linear dependency problems with the aug-cc-pVDZ and aug-cc-pVTZ basis sets when used for benzene.
Cyclic
water clusters are pivotal for understanding atmospheric
reactions as well as liquid water, yet the temperature (T) dependence of their dynamics and spectroscopy is poorly studied.
The development of highly accurate water potentials, such as MB-pol,
partly rectifies this. It remains to account for the quantum nuclear
effects (NQE), because quantum nuclear dynamics become increasingly
inaccurate at low temperatures. From a practical point of view, we
find that NQE can be accounted for simply by subtracting a constant
from the frequencies obtained from the velocity autocorrelation functions
(VACF) of classical nuclear dynamics, resulting in unprecedented agreement
with experiment, mostly within 5 cm–1. We have performed
classical simulations of (H2O)
n
clusters (n = 2–5) from 20 K and up to their
melting temperature, calculating both all-atom and partial VACF, thus
generating the temperature dependence of the vibrational frequencies
(IR and Raman bands). Focusing on the hydrogen-bonded (HBed) OH stretch
and HOH bend, we find opposing T dependencies. The
HBed OH modes blue shift linearly with T, attributed
to ring expansion rather than any specific conformational change.
The lowest-frequency Raman concerted mode is predicted to show the
largest such shift. In contrast, the HOH bend undergoes a red-shift,
with the highest frequency concerted band undergoing the largest red-shift.
These results can be explained by a coupled-oscillator model for n hydrogen atoms on a ring, constrained to move either tangentially
(stretch) or perpendicularly (bend) to the ring. With increasing temperature
and weakening of HBs, the intrinsic force constant increases (stretch)
or remains constant (bend), while the nearest-neighbor coupling constant
decreases, and this results in the interesting behavior revealed herein. T-dependent Raman studies are required for testing some
of these predictions.
In the search for replacement of the platinum‐based catalysts for fuel cells, MN4 molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum‐group metal‐free (PGM‐free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e− to 4e− pathway in the pH range of 3.5‐6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pKa values of the protonation sites in the vicinity of the ORR active sites. Control of the electron‐withdrawing nature of these groups characterized by the pKa values could switch the ORR from the H+ to e− rate‐determining step mechanisms and from 2e− to 4e− ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pKa of the N atoms at the active sites can be used as a descriptor for the design of high‐performance, durable PGM‐free catalysts.
Carbons are ubiquitous electrocatalytic supports for various energy-related transformations, especially in fuel cells. Doped carbons such as Fe–N–C materials are particularly active towards the oxidation of hydrazine, an alternative fuel...
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