Substituent Effects on Two-Center Three-Electron Bonds and Hydrogen Bonds Involving Unsaturated Organic Functional Groups and an Ammonia Radical Cation-The Resonance Contribution

Pyrene-Based Organic Photoredox Catalysts for Carbon-Carbon Bond-Forming Reactions: Reductive Coupling of Aromatic Carbonyl and Imine Compounds

Metal-free photoredox catalysts built upon a pyrene core were developed for carbon-carbon bond-forming reactions. Among these catalysts, a pyrene derivative containing a urea moiety effectively facilitated the reductive coupling of aromatic carbonyl and imine compounds under blue LED irradiation. This process provided the corresponding vicinal diols and diamines in good yields.

Development of Carbazole-Cored Organo-Photocatalyst for Visible Light-Driven Reductive Pinacol/Imino-Pinacol Coupling

Benzoperylenocarbazole (BPC), a unique carbazole-based organophotocatalyst, is reported herein as a potent organo-photoreductant. Lower excited state oxidation potential (-2.0 V vs SCE) and reasonable excited state lifetime (4.61 ns) render BPC an effective photosensitizer. Under irradiation of blue light employing low catalyst loading (0.5 mol %), a plethora of vicinal diols and diamines were synthesized in excellent yields through reductive coupling of carbonyls and imines, respectively. Insight about the electronic structure of BPC was obtained by DFT calculations.

Redshift or adduct stabilization--a computational study of hydrogen bonding in adducts of protonated carboxylic acids

It is generally expected that the hydrogen bond strength in a D-H(***)A adduct is predicted by the difference between the proton affinities (DeltaPA) of D and A, measured by the adduct stabilization and demonstrated by the infrared (IR) redshift of the D-H bond stretching vibrational frequency. These criteria do not always yield consistent predictions, as illustrated by the hydrogen bonds formed by the E and Z OH groups of protonated carboxylic acids. The DeltaPA and the stabilization of a series of hydrogen bonded adducts indicate that the E OH group forms the stronger hydrogen bonds, whereas the bond length changes and the redshift favor the Z OH group, matching the results of NBO and AIM calculations. This reflects that the thermochemistry of adduct formation is not a good measure of the hydrogen bond strength in charged adducts, and that the ionic interactions in the E and Z adducts of protonated carboxylic acids are different. The OH bond length and IR redshift afford the better measure of hydrogen bond strength.

Proton-Bound Homodimers: How Are the Binding Energies Related to Proton Affinities?

High-level quantum chemical calculations [G3(MP2)-RAD//MP2/6-31+G(d,p)] have been employed to investigate the relationship between the binding energy (BE) of a substrate (X) and its protonated form [H-X]+ with the proton affinity (PA) of the substrate (X) in several series of protonated homodimers ([X...H-X]+). We find that for each series of closely related substrates, the binding energy (BE) is correlated with the proton affinity (PA) in an approximately quadratic manner. Thus, for a given series, the BE initially increases in magnitude with increasing PA, reaches a point of maximum binding, and then becomes smaller as the PA increases further. This behavior can be attributed to the competing effects of the exothermic partial protonation of the substrate and the endothermic partial deprotonation of the protonated substrate. As the PA increases, protonation of X contributes to increased binding but the penalty for partial deprotonation of [H-X]+ also increases. Once the PA becomes sufficiently high, the penalty for the partial deprotonation of [H-X]+ dominates, leading to maximum binding occurring at intermediate PA.

Complexes with N-H(+)-P hydrogen bonds: structures, binding energies, and spin-spin coupling constants

Ab-initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of proton-bound complexes stabilized by N-H+-P hydrogen bonds and to investigate the nature of the proton-transfer coordinate in these systems. Double minima are found only when the difference between the protonation energies of the N and P bases is less than about 4 kcal/mol. The isomer in which the protonated nitrogen base is the donor lies lower on the potential surface and also has a greater binding energy relative to the corresponding isolated monomers. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been employed to obtain one- and two-bond spin-spin coupling constants across these hydrogen bonds. Two-bond coupling constants (2h)J(N-P) correlate with N-P distances, irrespective of whether the donor ion is N-H+ or P-H+. One-bond coupling constants (1)J(N-H) and (1h)J(H-P) for complexes stabilized by N-H+...P hydrogen bonds correlate with corresponding distances, but similar correlations are not found for (1)J(P-H) and (1h)J(H-N) for complexes with P-H+...N hydrogen bonds. Negative values of (1h)K(H-N) and (1h)K(H-P) indicate that the hydrogen bonds in these complexes are traditional. Comparisons are made with complexes stabilized by N-H+-N and P-H+-P hydrogen bonds.

Theoretical Study of (CH···C)- Hydrogen Bonds in CH4-nXn(X = F, Cl; n = 0, 1, 2) Systems Complexed with Their Homoconjugate and Heteroconjugate Carbanions

This work deals with a theoretical study of the (CH...C)- hydrogen bonds in CH4, CH3X, and CH2X2 (X = F, Cl) complexed with their homoconjugate and heteroconjugate carbanions. The properties of the complexes are calculated with the B3LYP method using the 6-311++G(d,p) or 6-311++G(2df,2p) basis sets. The deprotonation enthalpies (DPE) of the CH bond or the proton affinities of the carbanions (PA(C-) are calculated as well. All the systems with the exception of the CH4...CHCl2(-) one are characterized by a double minimum potential. In some of the complexes, the (CH(b)...C)- hydrogen bond is linear. In other systems, such as CH3F...CH2F- and CH3F...CHF2(-), there is a large departure from linearity, the systems being stabilized by electrostatic interactions between the nonbonded H of the neutral molecule and the F atom of the carbanion. In the transition state, the (CH(b)...C)- bond is linear, and there is a large contraction of the intermolecular C...C distance. The binding energies vary within a large range, from -1.4 to -11.1 kcal mol(-1) for the stable complexes and -8.6 to -44.1 kcal mol(-1) for the metastable complexes. The energy barriers to proton transfer are between 5 and 20 kcal mol(-1) for the heteroconjugate systems and between 3.8 and 8.3 kcal mol(-1) for the homoconjugate systems. The binding energies of the linear complexes depend exponentially on 1.5DPE - PA(C-), showing that the proton donor is more important than the proton acceptor in determining hydrogen bond strength. The NBO analysis indicates an important electronic reorganization in the two partners. The elongations of the CH bond resulting from the interaction with the carbanion depend on the occupation of the sigma*(CH(b)) antibonding orbitals and on the hybridization of the C bonded to H(b). The frequency shifts of the nu(CH)(A1) stretching vibration range between 15 and 1150 cm(-1). They are linearly correlated to the elongation of the CH(b) bond.

Theoretical study of the symmetry of the (OH...O)- hydrogen bonds in vinyl alcohol-vinyl alcoholate systems

The interactions between substituted vinyl alcohols and vinyl alcoholates (X = NH(2), H, F, Cl, CN) are studied at the B3LYP/6-311++G(d,p) level of theory. In a first step, the conformation of the monomers is investigated and the proton affinities (PA(A(-))) of the enolates are calculated. The enols and enolates are held together by strong (OH...O)(-) hydrogen bonds, the hydrogen bond energies ranging from 19.1 to 34.6 kcal mol(-1). The optimized O...O distances are between 2.414 and 2.549 A and the corresponding OH distances from 1.134 and 1.023 A. The other geometry parameters such as C[double bond]C or CO distances also indicate that, in the minimum energy configuration, the hydrogen bonds are characterized by a double well potential. The Mulliken charges on the different atoms of the proton donors and proton acceptors and the frequencies of the nu(OH) stretching vibrations agree with this statement. All the data indicate that the hydrogen bonds are the strongest in the homomolecular complexes. The transition state for hydrogen transfer is located with the transition barrier estimated to be about zero. Upon addition of the zero-point vibration energies to the total potential energy, the barrier vanishes. This is a characteristic feature of low-barrier hydrogen bonds (LBHBs). The hydrogen bond energies are correlated to the difference 1.5 PA(AH) - PA(A(-)). The correlation predicts different energies for homomolecular hydrogen bonds, in agreement with the theoretical calculations. Our results suggest that a PA (or pK(a)) match is not a necessary condition for forming LBHBs in agreement with recent data on the intramolecular hydrogen bond in the enol form of benzoylacetone (J. Am. Chem. Soc. 1998, 120, 12117).

Methyl substituent effects in [H(n)X...XH(n)](+) three-electron-bonded radical cations (X = F, O, N, Cl, S, P; n = 1 - 3). An ab initio theoretical study

The effects of methyl substitution on the geometries and bonding energies of a systematic series of three-electron-bonded radical cations of the type [H(n)X...XH(n)](+), covering all possible symmetrical three-electron bonds that may take place between atoms of the second and third rows of the periodic table, have been investigated at the level of Møller-Plesset perturbation theory. Methyl substitution leads to significant weakening and lengthening of the X...X bond when X is a second-row atom. The effects increase with the number of substitutions and are more and more important in the series X = N, O, F. By contrast, methyl substitution leaves the bonding energies between third-row atoms practically unchanged but leads to a surprising bond shortening in the S...S and P...P cases. These seemingly contradictory effects are rationalized through a qualitative analysis based on an elementary molecular orbital description of three-electron bonding.