Orbital relaxation and the third-order induction energy in symmetry-adapted perturbation theory

Theoretical investigations of the induction interaction between closed-shell molecules which fully account for the orbital relaxation effects are presented. Explicit expressions for the third-order induction energy in terms of molecular integrals and orbital energies are given and implemented within the sapt2008 program for symmetry-adapted perturbation theory (SAPT) calculations. Numerical investigations for the He–He, He–LiH, Ar–Ar, H₂–CO, H₂O–H₂O, and H₂O–NH₃ model dimers show that the orbital relaxation increases the third-order induction interaction by 15 to 50% at near-equilibrium geometries, with the largest effect observed for complexes involving highly polar monomers. At large intermonomer separations, the relaxed third-order induction energy perfectly recovers the difference $\delta E^{\rm HF}_{\rm int}$ between the Hartree–Fock interaction energy and the sum of the uncorrelated SAPT contributions through second order in the intermolecular interaction operator. At the near-equilibrium geometries, the sum of the relaxed third-order induction and exchange-induction energies reproduces, however, only a small fraction (6 to 15%) of $\delta E^{\rm HF}_{\rm int}$ for the nonpolar systems and about 40 to 60% for the polar ones. A comparison of the complete SAPT calculations with the coupled-cluster treatment with single, double, and noniterative triple excitations [CCSD(T)] suggests that the pure SAPT approach with all the available third-order corrections is more accurate for nonpolar systems while for the polar ones the hybrid approach including $\delta E^{\rm HF}_{\rm int}$ gives better results.     

Structure and atropisomerisation of new diastereomeric gossypol Schiff bases with (R)-(+)-2-amino-3-benzyloxy-1-propanol studied by NMR,ECD and DFT methods

Gossypol Schiff base with (R)-(+)-2-amino-3-benzyloxy-1-propanol 1 was synthesised and resolved by HPLC method into diastereomers to study their atropisomerisation process. The spectroscopic analysis performed by one- and two-dimensional NMR, UV–vis and FT-IR methods indicated that the compound exists in solution as an enamine-oxo tautomer. The ECD measurements and TD-DFT calculations allowed us to unambiguously determine the configuration about the axially chiral biaryl moiety of 1. The conditions of the atropisomerisation processes of diastereopure gossypol Schiff bases (S_AX,R)-1 and (R_AX,R)-1 were determined on the basis of ECD and NMR measurements. Exposure of the diastereomers of 1 to sunlight and to the light at λ = 254 nm significantly accelerated the atropisomerisation when compared to its rate in the dark.     

Spectroscopic and PM5 semiempirical studies of new hydrazone of gossypol with 3,6-dioxaheptylhydrazine

A new hydrazone of gossypol with 3,6-dioxaheptylhydrazine (GHDO) has been synthesised and its structure has been studied by FT-IR, ¹H NMR, ¹³C NMR as well as PM5 semiempirical methods. All the studies have provided clear evidence of the existence of GHDO in the solution in the N-imine–N-imine tautomeric form. The structure and the spectroscopic behaviour of this tautomer are discussed in details. It is shown the structure of GHDO is strongly stabilised by different types of intramolecular hydrogen bonds. In two of them the oxygen atoms of the oxaalkyl chains are also engaged. The strongest intramolecular hydrogen bond is formed between the O₇H proton and N₁₆ atom from the hydrazone group.     

Distance-dependent quenching of Nile Blue fluorescence byN,N-diethylaniline observed by frequency-domain fluorometry

Fluorescence quenching of Nile Blue by amines is thought to be due to electron transfer to the excited dye molecule from the amine electron donor. We used electron transfer quenching of Nile blue byN,N-diethylaniline in propylene glycol as a model system for an interaction which depends exponentially on distance. We investigated the time dependence of the presumed distance-dependent process using gigahertz harmonic-content frequency-domain fluorometry. The frequency-domain data and the steady-state quantum yield were analyzed globally based on either the Smoluchowski-Collins-Kimball radiation boundary condition (RBC) model or the distancedependent quenching (DDQ) model, in which the rate of quenching depends exponentially on the flourophore-quencher distance. We performed a global analysis which included both the frequencydomain time-resolved decays and the steady-state intensities. The latter were found to be particularly sensitive to the model and parameter values. The data cannot be satisfactorily analyzed using the RBC model for quenching. The analysis shows the excellent agreement of the DDQ model with the experimental data, supporting the applicability of the DDQ model to describe the quenching by the electron transfer process, which depends exponentially on the donor-acceptor distance.     

1H NMR studies of intramolecular hydrogen bonding in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of seven N-(pyridyl)amides of 6-methylpicolinic acid N-oxide in chloroform were obtained. The influence on the chemical shifts of the N? H protons of temperature, concentration and the CH₃ substituent in the pyridine ring was studied. The N? H protons were found to be shifted to low fields (～14 ppm) owing to the formation of strong intramolecular hydrogen bonding. The influence of the pyridine ring on the chemical shift of the N? H proton is comparable with the inductive effect of the p-nitrophenyl group. The hindered rotation around the N-pyridyl bond of N-(α-pyridyl)amides of 6-methylpicolinic acid in solution is discussed.     

1H n.m.r. studies on intramolecular hydrogen bonding in anilides of 6-methylpicolinic acid N-oxide

The ¹H n.m.r. spectra of eleven anilides of 6-methyl-picolinic acid N-oxide in chloroform were obtained. The influence of temperature, concentration and substituents on the chemical shifts of the N? H protons was investigated. The structure of the anilides is discussed.     

Spectroscopic and semiempirical studies of a proton channel formed by the methyl ester of monensin A

Monensin A is an ionophore able to carry protons and cations through the cell membrane. Its methyl ester (MON1) and its hydrates have been studied in acetonitrile, and its deuterated analogue by Fourier transform infrared (FTIR) and (1)H and (13)C NMR spectroscopies as well as by vapor pressure osmotic and PM5 semiempirical methods. Interestingly, these hydrates show new and unexpected biophysical and biochemical properties. The formation of the hydrates starts with a transfer of a proton from the O(IV)-H hydroxyl group of MON1 to an oxygen atom of a water molecule, which is subsequently hydrated by other water molecules forming the (MON1 + 3H(2)O) species. This hydrate exhibits a ringlike structure in which the water molecules form an almost linear hydrogen-bonded chain. Within this chain, the excess proton fluctuates very fast inside the water cluster as indicated by a continuous absorption in the FTIR spectra. The formation of the (MON1 + 3H(2)O) species is accompanied by a self-assembly process, leading to the formation of a proton channel made up of eight (MON1 + 3H(2)O) units with a length of 60 A, in which the proton can fluctuate over the whole distance. Semiempirical calculations suggest that due to the hydrophobic surface the channel can be incorporated readily in a lipid bilayer. This hypothetical new channel is thought to be able to transport protons through the cell membrane. Thus it is a suitable model for studying proton-transfer processes, and in addition, it may open interesting new fields of application.     

Degenerate symmetry-adapted perturbation theory of weak interactions between closed- and open-shell monomers: application to Rydberg states of helium hydride

Symmetry-adapted perturbation theory is extended to the (quasi) degenerate, open-shell case. The new formalism is tested in calculations of the interaction energies for a helium atom in the ground state interacting with an excited hydrogen atom. It is shown that the method gives satisfactory results if the coupling with higher Rydberg states of the dimer is small, as is the case for the A²Σ⁺,B²Π,E²Π,3²Π, and 1²Δ states of HeH. For the C²Σ⁺ state convergence of the method is very slow, but it can be improved by including the n=3 states in the model space. Received: 3 June 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998     

Phosphate Assisted Proton Transfer in Water and Sugar Glasses: A Study Using Fluorescence of Pyrene-1-carboxylate and IR Spectroscopy

The role of water’s H-bond percolation network in acid-assisted proton transfer was studied in water and glycerol solutions and in sugar glasses. Proton transfer rates were determined by the fluorescence of pyrene-1-carboxylate, a compound with a higher pK in its excited state relative to the ground state. Excitation of pyrene-1-COO⁻ produces fluorescence from pyrene-1-COOH when a proton is accepted during the excited singlet state lifetime of pyrene-1-COO⁻. The presence of glycerol as an aqueous cosolvent decreases proton transfer rates from phosphoric and acetic acid in a manner that does not follow the Stokes relationship on viscosity. In sugar glass composed of trehalose and sucrose, proton transfer occurs when phosphate is incorporated in the glass. Sugar glass containing phosphate retains water and it is suggested that proton transfer requires this water. The infrared (IR) frequency of water bending mode in sugar glass and in aqueous solution is affected by the presence of phosphate and the IR spectral bands of all phosphate species in water are temperature dependent; both results are consistent with H-bonding between water and phosphate. The fluorescence results, which studied the effect of cosolvent, highlight the role of water in assisting proton transfer in reactions involving biological acids, and the IR results, which give spectroscopic evidence for H-bonding between water and phosphate, are consistent with a mechanism of proton transfer involving H-bonding. The possibility that the phosphate-rich surface of membranes assists in proton equilibration in cells is discussed.     

DISTANCE-DEPENDENT FLUORESCENCE QUENCHING OF TRYPTOPHAN BY ACRYLAMIDE

Abstract We used GHz frequency-domain fluorometry to investigate the time-dependent intensity decays of N -acetyl -L-trytophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol at 20°C. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins-Kimball radiation boundary condition (RBC) model for quenching. The steady-state Stern-Volmer plots show significant upward curvature. At low temperature in vitrified propylene glycol (-60%), where translational diffusion cannot occur during the lifetime of the excited state, quenching of NATA by acrylamide was observed. The Smoluchowski and RBC quenching models do not predict any quenching in the absence of translational diffusion. Hence, these frequency-domain and steady-state data indicate a through-space quenching interaction between NATA and acrylamide. The rate for quenching of NAT A by acrylamide appears to depend exponentially on the fluorophore-quencher separation distance. Comparison of the time-resolved and steady-state data provides a sensitive method to determine the distance dependence of the fluorophore-quencher interaction. The distance-dependent rate of quenching also explains the upward curvature of the Stern-Volmer plot, which is often observed for quenching by acrylamide. These results suggest that the distance-dependent quenching rates need to be considered in the interpretation of quenching data of proteins by acrylamide.