Proton Affinity Correlations between Hydrogen and Dihydrogen Bond Acceptors

IR spectrum of SiH3OH2+SiH4: cationic OH⋯HSi dihydrogen bond versus charge-inverted SiH⋯Si hydrogen bond

The low electronegativity of Si gives rise to a variety of nonconventional intermolecular interactions in clusters of silanes and their derivatives, which have not been well characterized yet. Herein, we characterize the structures of various isomers of bare and Ar-tagged SiH3OH2+SiH4 dimers composed of protonated silanol and silane by infrared photodissociation (IRPD) of mass-selected ions and dispersion-corrected density functional calculations (B3LYP-D3/aug-cc-pVTZ). The analysis of the IRPD spectra recorded in the OH stretch range reveals the competition between two types of nonconventional hydrogen bonds (H-bonds). The first one represents a OH⋯HSi ionic dihydrogen bond (DHB), in which SiH4 interacts with the H2O moiety of SiH3OH2+. The second one represents a charge-inverted SiH⋯Si ionic H-bond (CIHB), in which the SiH4 ligand interacts with the SiH3 moiety of SiH3OH2+. The latter may also be considered as a weak three-centre two-electron (3c-2e) bond. Although both types of H-bonds are computed to have comparable interaction strengths for SiH3OH2+SiH4 (D0 ≈ 35-40 kJ mol-1), DHB isomers dominate the population in the supersonic plasma expansion, while the abundance of CIHB isomers is roughly one order of magnitude lower, probably as a result of entropic factors.

Excited-State Dynamics Affected by Switching of a Hydrogen-Bond Network in Hydrated Aminopyrazine Clusters

The cluster structures of hydrated aminopyrazines, APz-(H₂O)_n=2-4, in supersonic jets have been investigated measuring the size-selected electronic and vibrational spectra and determined with the aid of quantum chemical calculations. The APz-(H₂O)₂ structure is assigned as a cyclic N1 type where a homodromic hydrogen-bond chain starts from the amino group and ends at the 1-position nitrogen atom of the pyrazine moiety, corresponding to 2-aminopyridine-(H₂O)₂. On the other hand, APz-(H₂O)_n=3,4 has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3-aminopyridine-(H₂O)_n=3,4. The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S₁-S₀ electronic transition that are entirely consistent with those of the corresponding 2-aminopyridine and 3-aminopyridine clusters and also shows the drastically strengthened fluorescence intensity of origin bands in the electronic spectrum. The significant change in the excited-state dynamics is explored by the fluorescence lifetime measurement and the time-dependent density functional theory (TD-DFT) calculation. It is suggested that the drastic elongation of fluorescence lifetimes is due to the change in the electronic structure of the first excited state from nπ* to ππ*, resulting in the decreasing spin-orbit coupling to T₁ (ππ*).

A comprehensive infrared spectroscopic and theoretical study on phenol-ethyldimethylsilane dihydrogen-bonded clusters in the S0 and S1 states

To investigate microscopic characters of Si-H⋯H-O type dihydrogen bonds, we observed OH and SiH stretch bands in both the S₀ and S₁ states of phenol-ethyldimethylsilane (PhOH-EDMS) clusters by infrared (IR)-ultraviolet (UV) and UV-IR double resonance spectroscopies. Density functional theory (DFT) calculations and energy decomposition analysis were also performed. Structures of two isomers identified were unambiguously determined through the analysis of IR spectra and DFT calculations. To discuss the strength of dihydrogen bond in various systems, we performed theoretical calculations on clusters of EDMS with several acidic molecules in addition to PhOH. It was revealed that charge-transfer interaction energies from a bonding σ orbital of SiH bond to an anti-bonding σ^* orbital of OH bond well reflect strengths of dihydrogen bonds. Additionally, it was found that the red shift of SiH stretch frequencies can be used as a crude measure of the strength of dihydrogen bonds. Relationship between the red shifts of OH/SiH stretch frequencies and various electrostatic components of the interaction energy was examined. In the S₁ state, large increases in red shifts were observed for both the OH and SiH stretch frequencies. Since the EDMS moiety is not associated with the electronic excitation in a cluster, the strength of dihydrogen bonds in the S₁ and S₀ states was able to be directly compared based on the red shifts of the SiH stretch bands. A significant increase in the red shift of SiH stretch frequency indicates a strengthening of the dihydrogen bonds during the electronic excitation of the PhOH moiety.

Impact of donor acidity and acceptor anharmonicity on vc=o spectral shifts in O-H···O=C H-bonded ketone-alcohol complexes: An IR spectroscopic investigation

O-H···O=C Hydrogen bonding (H-bonding) results in spectral shifts in both ν_O-H and ν_C=O modes. A large number of investigations exist in literature that focuses on how the spectral shifts vary with certain properties of the donors and acceptors. However information on how the magnitude of spectral shift is dictated individually by the donor and acceptor is not yet clear to us. Here, IR spectroscopy in room temperature CCl₄ solution has been used to investigate how ν_C=O spectral shifts of ketones in H-bonded complexes with alcohols are influenced by change in donor and acceptor properties. For this purpose 25 number of O-H···O=C H-bonded complexes formed by 5 different ketones and 5 alcohols have been studied. The magnitude of red-shifts shown by the ν_C=O bands were found to show systematic trends with changing donor and acceptor species. It was found that for a particular ketone, the magnitude of shift increases monotonically with acidity of the alcohols. Spectral shifts were found to increase in a linear fashion with decreasing p^K_a of the donor alcohols. On the other hand, when the alcohol were kept fixed, the spectral shift was found to be dependent on the CO bond strength of the ketones. We found spectral shifts to linearly increase with increasing anharmonicity constant and decreasing dissociation energy of CO bond. Finally, it has been shown that there exist concomitant correlations of the spectral shifts with donor p^K_a and acceptor ν_C=O anharmonicity constant/bond dissociation energy. The relations have been validated for H-bonded complexes of 1,4-cyclohexanedione with the above mentioned five alcohols.

Electrostatics and Dispersion in X-H···Y (X = C, N, O; Y = N, O) Hydrogen Bonds and Their Role in X-H Vibrational Frequency Shifts

The frequency shifts of donor stretching vibration in X-H···Y (X = C, N, O; Y = N, O) hydrogen-bonded complexes of phenylacetylene, indole, and phenol are linearly correlated with the electrostatic component of the interaction energy. This linear correlation suggests that the electrostatic component, which is the first-order perturbative correction to the stabilization energy, is essentially localized on the X-H group. The linear correlation suggests that the electrostatic tuning rate, which is a measure of the X-H oscillator to undergo shifts upon hydrogen bonding per unit increase in the electrostatic component of the stabilization energy, was found to be in the order of O-H > N-H > C-H. Interestingly, for each of the donor groups, viz., C-H, N-H, and O-H, the vibrational frequency shifts were inversely correlated to the dipole moment of the acceptor separately, which is counterintuitive vis-à-vis the electrostatic component. This implies that extrapolation to zero dipole moment of the acceptor will yield very large shifts in the hydrogen-bonded X-H stretching frequencies. The trends in the variation of the dispersion and exchange-repulsion components and the total interaction energy vis-à-vis frequency shifts of donor stretching vibration are similar for hydrogen-bonded complexes of phenylacetylene, indole, and phenol. Furthermore, it was observed that the vibrational frequency shifts of all of the complexes are linearly correlated with the charge transfer from the filled orbital of the hydrogen acceptor to the vacant antibonding (σ*) orbital of the X-H donor group on the basis of natural bonding orbital calculations.

Theoretical study on the dehydrogenation reaction of dihydrogen bonded phenol–borane-trimethylamine in the excited state

No dehydrogenation reaction occurs in the ground state of dihydrogen bonded phenol–BTMA. TS-S₁₀ points to the formation of a hydrogen molecule, while TS-S₁₁ points to the B atom. The dehydrogenation reaction along TS-S₁₀ is energy favorable, unlike that along TS-S₁₁.

Electrostatics determine vibrational frequency shifts in hydrogen bonded complexes

The shifts in the acetylenic C–H stretching vibration in the C–H⋯X hydrogen-bonded complexes correlate with the electrostatic component of the stabilization energy.

The B-H...H-P dihydrogen bonding in ion pair complexes [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)] (n = 0-3) and its implication in H(2) elimination and activation reactions

The B-H(delta-)...(delta+)H-P dihydrogen bonding (DHB) in ion pair complexes [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)] (n = 0-3) and its role in the combination of proton and hydride with the release of H(2) or, reversibly, the heterolytic activation of H(2) by Lewis pairs (CF(3))(3)BPH(3-n)(Me)(n) have been theoretically investigated at the MP2 and DFT levels. It is found that the B-H...H-P bonds behave similarly to those in neutral pairs and ion-molecule complexes in most respects, such as the linearity of the H...H-P moiety, the characteristics of the electron transfer and rearrangement, and the topological properties of the DHB critical point, except that in certain cases, a blue-shifting of the H-bond vibrational frequency is observed. In [(CF(3))(3)BH(-)][HPH(3-n)(Me)(n)(+)], the proton shifting within the complexes leads to the formation of the dihydrogen complex B(CF(3))(3)(eta(2)-H(2)), which is followed by a subsequent H(2) release. The stability of B(CF(3))(3)(eta(2)-H(2)) (D(e)/D(0) = 10.8/6.0 kcal/mol) makes the proton-hydride combination proceed in a fashion similar to the protonation reactions in transition-metal hydrides rather than those in group 13 hydrides EH(4)(-) (E = B, Al, Ga). As for the H(2)-splitting reaction R(3)BPR'(3) + H(2) --> [R(3)BH(-)][HPR'(3)(+)], classical Lewis pair (CLP) (CF(3))(3)BPH(3) exhibits a high barrier and results in an unstable ion pair product [(CF(3))(3)BH(-)][HPH(3)(+)] compared with the "frustrated Lewis pair" (FLP) (C(6)F(5))(3)BP(tBu)(3). A detailed analysis of the mechanistic aspects of H(2) activation by (CF(3))(3)BPH(3) and (C(6)F(5))(3)BP(tBu)(3), supported by another CLP (CF(3))(3)BP(tBu)(3) which has a binding energy comparable to (CF(3))(3)BPH(3) but a reaction exothermicity comparable to (C(6)F(5))(3)BP(tBu)(3), allows us to suggest that the low stability of FLP (C(6)F(5))(3)BP(tBu)(3) is the determining factor for the low reaction barrier. The relative stability and other properties of the ion pair products [R(3)BH(-)][HPR'(3)(+)] have also been analyzed. Results strongly support the view proposed by Rokob et al. [ Rokob , T. A. ; Hamza , A. ; Stirling , A. ; Soos , T. ; Papai , I. Angew. Chem., Int. Ed. 2008 , 47 , 2435 ] that the frustration energy lowers the energy barrier and increases the exothermicity of the reaction.

Photoelectron spectroscopy of hydrated hexafluorobenzene anions

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6(-)(H2O)n (n=0-2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6(-)(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O-H...F ionic hydrogen bonds. Whereas in the case of C6F6(-)(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6(-)(H2O)1.