Hydrogen-bond between the oppositely charged hydrogen atoms It was suggested by crystal structure analysis. A small number of spectroscopic studies have been reported. X−H···H−Y -- ++ ++ -- X = O, N, … Y = B, Li, Al, … Dihydrogen-bond (DHB) G.N. Patwari et al. J. Chem. Phys. 113, 9885 (2000). J. Chem. Phys. 114, 8877 (2001).

Phenol(PhOH)- Diethylmethylsilane(DEMS) system Infrared spectroscopy OH stretch band of PhOH ( OH ) Small red-shift of OH (20 − 30 cm -1 ) Competition between the dihydrogen-bond and the dispersion force. Ref. H. Ishikawa et al. J. Chem. Phys. 123, (2005). Previous study Dihydrogen-bond + Dispersion

PhOH-Triethylsilane(TES) system - Distinct type of isomer Larger contribution of the Dihydrogen bond Previous study DHB + Dispersion DHB?

To reveal the intrinsic character of the Si-H∙∙∙H-O dihydrogen-bond, dihydrogen-bonded clusters in which the dihydrogen bond is the dominant interaction should be examined. PhOH + -DEMS, PhOH + -TES Large acidity ∙∙∙ Enhancement of DHB (  ) -1 configuration ∙∙∙ Weakening of the dispersion force Intrinsic character of the Si-H∙∙∙H-O DHB

Setup Experimental DEMS, TES / He UV PhOH IR Q-mass Mass spectrum 1. Clusters are generated by collisions of laser-ionized PhOH + with DEMS or TES at the exit of the nozzle. 2. IR laser light is irradiated on the mass-selected cluster ions. 3. IR absorption is detected as a production of the IR photo-fragmentation.

IR spectra of PhOH + -DEMS/TES Large red-shift of OH compared with that of PhOH + (3534 cm -1 ). Ref. A. Fujii et al. J. Phys. Chem. 106, (2002). Broad width implies a strong anharmonicity of OH stretch mode. Several isomers with respect to the internal rotation of ethyl groups may be involved. PhOH + -DEMS PhOH + -TES C-H PhOH +

PhOH + -TES OH (calc.) = 2937 cm -1 cf. OH (obs.) = 2855 cm -1 R(H···H) = Å, R(OH) = Å D 0 = 50.0 kJ mol -1 = 4183 cm -1 Gaussian 09, M05-2X/ G(3d,2p) Density functional theory calculation Dihydrogen-bond interaction is dominant −0.294

It is interesting to estimate contributions of these intermolecular interactions. Comparison between the B3LYP and the M05-2X calculations is carried out. The dispersion interaction cannot be treated by the B3LYP functional. Dispersion vs. SiH···HO dihydrogen-bond M05-2XB3LYP

There is a large contribution of the dispersion interaction in stabilizing the neutral cluster. The character of the DHB should be close to its intrinsic one. The DHB is dominant in the cationic cluster. Dispersion vs. SiH···HO dihydrogen-bond PhOH-TESBinding energy  OH (obs. −78 cm −1 ) B3LYP5.0 kJ/mol−90 cm −1 M05-2X13.7 kJ/mol−70 cm −1 PhOH + -TESBinding energy  OH (obs. −674 cm −1 ) B3LYP47.5 kJ/mol−700 cm −1 M05-2X50.0 kJ/mol−597 cm −1

The amount of the red-shift of OH stretch frequency is a good indicator of the strength of the hydrogen-bond interaction. Comparison with the other PhOH-X 1:1 cluster. Intrinsic strength of the SiH···HO dihydrogen-bond is somewhat stronger than that of  ···HO system. Strength of the SiH···HO dihydrogen-bond N2N2 COTESC2H4C2H4 C6H6C6H6 H2OH2O PhOH-X−5−5−33−78−77−78−133 PhOH + -X−159−211−674−514−474−954 OH of PhOH/PhOH + -X 1:1 cluster

Proton affinity of M at 0 K is calculated as a binding energy between M and H +. Estimated PA’s are consistent with our observations. PA of TES and DEMS is larger than  -molecules. Proton affinity of TES and DEMS MCalcd.Ref. a TES193 DEMS188 C2H4C2H C6H6C6H Estimated proton affinities (kcal/mol) Ref. J. Phys. Chem. Ref. Data 27, 413 (1998).

Summary J. Phys. Chem. A, 119, (2015). Infrared spectroscopy of dihydrogen-bonded clusters are carried out to reveal the intrinsic character of the DHB involving the Si-H group. The  OH values indicates that the strength of the Si-H∙∙∙H-O DHB is found to be stronger than those of C 6 H 6 and C 2 H 4. Calculated proton affinity values are consistent with our observations. The high reactivity of Si-H hydrogen may be related to the large proton affinity.

IR spectra of DEMS and TES vapor Quite similar spectra with each other Differences in Si-H and rotational envelopes DEMS TES C-HSi-H

Investigations on hydrogen-bond hydrogen-bond - One of intermolecular interactions - Chemical reactions, structures and functions Spectroscopic characterization - Change in frequency and intensity of XH stretch - Infrared spectroscopy New types of hydrogen-bond CH-  type Dihydrogen-bond

IR spectrum of PhOH + -DEMS cluster Broad band accompanying several peaks appear below 3000 cm -1. Large redshift of OH compared with that of PhOH + (3534 cm -1 ). Ref. A. Fujii et al. J. Phys. Chem. 106, (2002). IR laser intensity Raw spectrum Normalized spectrum PhOH +

Gaussian 09, M06-2X/6-31++G(d,p) Natural bond orbital (NBO) analysis Strength of  -type hydrogen-bond is correlated with the donor-acceptor charge transfer interaction. Ref. Reed et al., Chem. Rev. 88, 899 (1988). Example: PhOH-H 2 O n orbital (donor)  * orbital (acceptor) 48.6 kJ mol −1 Gaussian 09, M05-2X/ G(3d,2p)

NBO analysis for PhOH + -DEMS Si-H  orbitalO-H  * orbital kJ mol − kJ mol −1 7.8 kJ mol −1 Si-H  * orbitalSi-C  * orbital

NBO analysis for PhOH + -DEMS Dihydrogen-bonding is the major interaction. Other interactions also appreciable contribution.

Most stable isomer OH (calc.) = 3644 cm -1 cf. OH (obs.) = 3633 cm -1 R(H ··· H) = 2.24 Å,  (CCOH) = 15.2° D 0 = 16.4 kJ mol -1 =1370 cm -1 Gaussian 09, M05-2X/ G(3d,2p) Density functional theory calculation Dihydrogen-bond and Dispersion force (CH-  ) −

Gaussian 09, M05-2X/ G(3d,2p) Density functional theory calculation Possible structure for isomer D OH (calc.) = 3587 cm -1 cf. OH (obs.) = 3578 cm -1 R(H ··· H) = 1.96 Å,  (CCOH) = 3.6° D 0 = 13.7 kJ mol -1 = 1145 cm -1 Large contribution of the dihydrogen-bond Small overlap −0.238