Spatial resolution of tip-enhanced Raman spectroscopy - DFT assessment of the chemical effect.

Experimental evidence of extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been recently demonstrated. Here, we present a full quantum chemical description (at the density functional level of theory) of the non-resonant chemical effects on the Raman spectrum of an adenine molecule mapped by a tip, modeled as a single silver atom or a small silver cluster. We show pronounced changes in the Raman pattern and its intensities depending on the conformation of the nanoparticle-substrate system, concluding that the spatial resolution of the chemical contribution of TERS can be in the sub-nm range.

Origin of the Regioselectivity in the Gas-Phase Aniline+CH3 (+) Electrophilic Aromatic Substitution.

Nonadiabatic ab initio molecular dynamics simulations are carried out to monitor the attack of CH3 (+) on aniline in the gas phase to form the corresponding σ complexes. The reaction is ultrafast and is governed by a single electron transfer within 30 fs, which involves two sequential conical intersections and finally produces a radical pair. Positive-charge allocation in the aromatic compound is found to govern the substitution pattern in ortho, meta, or para position.

Discrimination of nuclear spin isomers exploiting the excited state dynamics of a quinodimethane derivative.

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed.

Gas-phase electrophilic aromatic substitution mechanism with strong electrophiles explained by ab initio non-adiabatic dynamics.

Ab initio non-adiabatic dynamics is used to monitor the attack of CH3(+) to benzene. The results show that in the gas phase the reaction is ultrafast and is governed by a single electron transfer producing a radical pair.

Stark control of a chiral fluoroethylene derivative.

Hydrogen dissociation is an unwanted competing pathway if a torsional motion around the C═C double bond in a chiral fluoroethylene derivative, namely (4-methylcyclohexylidene) fluoromethane (4MCF), is to be achieved. We show that the excited state H-dissociation can be drastically diminished on time scales long enough to initiate a torsion around the C═C double bond using the nonresonant dynamic Stark effect. Potential energy curves, dipoles, and polarizabilities for the regarded one-dimensional reaction coordinate are calculated within the CASSCF method.

H-abstraction is more efficient than cis-trans isomerization in (4-methylcyclohexylidene) fluoromethane. An ab initio molecular dynamics study.

Non-adiabatic molecular dynamics simulations have been performed in the fluoro-olefin (4-methylcyclohexylidene) fluoromethane (4MCF) using multiconfigurational CASSCF (complete active space self-consistent field) on-the-fly calculations. As an olefin containing a C[double bond, length as m-dash]C double bond, 4MCF is expected to undergo cis-trans isomerization after light irradiation. However, ab initio molecular dynamics shows that a preferential dissociation of atomic hydrogen is taking place after population transfer to the bright ππ* state.

Quantum chemical investigation of the thermal rearrangement of cis- and trans-pinane.

The thermal rearrangement reactions of cis-pinane, 1, and trans-pinane, 2, into beta-citronellene, 3, and isocitronellene, 4, have been investigated using ab initio multiconfigurational CASSCF and CASSCF MP2 calculations. Concerted as well as stepwise retro-[2+2]-cycloaddition conversion mechanisms are discussed and the corresponding stationary points along the relevant reaction paths from the bicyclic starting compounds into their acyclic isomers have been optimized. Our calculations show that the stepwise retro-[2+2]-cycloaddition via biradicals is energetically favoured with respect to the concerted mechanism.

Kinetic model for the thermal rearrangement of cis- and trans-pinane.

On the basis of pyrolysis experiments with cis-pinane ( 1a), trans-pinane ( 1b), beta-citronellene ( 2), and isocitronellene ( 3), rate constants and activation parameters for the thermal rearrangement of the title compounds were calculated. Combining these with experimental parameters (residence time) allows for the kinetic modeling of the thermal rearrangement of 1a, 1b, 2, and 3. The chosen model of competitive first-order reactions describes the thermal behavior of the title compounds in a very good manner over a wide temperature range.