Sergei N. Smirnov

NUCLEAR SCALAR SPIN - SPIN COUPLING REVEALS NOVEL PROPERTIES OF LOW-BARRIER HYDROGEN BONDS IN A POLAR ENVIRONMENT

The structure of the hydrogen bridge 19 F· ·· 1 H· ·· 15 N in the acid - base complex A ··· H ··· B formed by HF and ( 15 N)2,4,6-trimethylpyridine in CDF3/ CDF2Cl has been studied between 112 K and 200 K by low-temperature, multinuclear NMR spectroscopy. For the first time scalar spin - spin coupling between all three nuclei of a hydrogen bridge is observed. This bridge exhibits a two-bond coupling constant 2 J19F15N of about 96 Hz, which is larger than the one-bond coupling constants 1 J1H15N and 1 J19F1H. The latter are strongly dependent on temperature. The function 1 J1H15Na f( 1 J19F1H) cannot be described in terms of a conventional equilibrium between the molecular and the zwitterionic form, but only with the intermediate forma- tion of very strongly hydrogen-bonded complexes of the type A dˇ ··· H· ·· B da that exhibit a vanishing or very small barrier for the proton motion. Here, the difference between the covalent bond and the hydrogen bond disappears even in the case of a polar solvents, as indicated by the large value of 2 J19F15N. Implications for the mechanism of pro- ton transfer and of acid - base catalyzed enzyme reactions in a locally aprotic but polar environment are discussed.

H/D isotope effects on the low-temperature NMR parameters and hydrogen bond geometries of (FH)2F− and (FH)3F− dissolved in CDF3/CDF2Cl

Using liquid state 1H, 2H and 19F NMR spectroscopy in the temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)2F− and (FH)3F− and their partially and fully deuterated analogs dissolved in the low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)4N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)2F−, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)3F− have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.

The role of short hydrogen bonds in mechanisms of enzymatic action

Abstract 1 H and 13 C NMR spectra of trypsin and ribonuclease, stabilized by chemical modification with a hydrophilic polymer, have been obtained over a wide pH range (1–11). The spectral features, referred to some nuclei of the catalytic sites (the “catalytic triad” for trypsin and the His-12—His-119 pair for ribonuclease), have been identified using different NMR techniques as well as chemical modification with selective reagents. It is found that monoprotonation of these systems leads to symmetrical (or quasi-symmetrical) H-bonds formed between the basic groups. This allows us to explain the discrepancies between experimental data obtained by different authors on the protonation sites in these catalytic systems. The simulation of the catalytic triad by a 15 N labeled low molecular weight model has led us to the conclusion that external agents do not cause any discrete proton transfers but do cause a smooth shift of the bridging protons from one basic atom to another, with the quasi-symmetrical H-bonds being formed in intermediate cases. On the basis of these experimental data, a new concept has been proposed for the mechanism of acid—base catalysis performed by the pairs of weak basic groups like His—Im and Asp(Glu)—COO − (p K a 3—7) which are not capable of proton abstraction from alcoholic or water OH groups (p K a > 13). This catalysis may consist on the one hand of changing the charge densities on reacting groups due to strong H-bonding and, on the other hand, of facilitating the free movement of a proton in the field of several basic atoms when going along the reaction coordinate. The energy of the very strong H-bonds thus formed diminishes the activation energy of the reaction.

1H NMR spectra and internal rotation of the OH-group in 2,6-Bis-(diethylaminomethyl)-phenols

Abstract 1 H NMR spectra (200 MHz) of solutions containing various 4-X-2,6-bis-(diethylaminomethyl)-phenols (X= OCH 3 , Cl, COOEt, NO 2 ) have been obtained in the temperature range 150 – 250 K. The signals of 3,5-aromatic protons as well as those of 2,6-CH 2 -protons have been found to undergo doublet splitting due to slowing down internal rotation of the OH-group at 170 – 190 K. The frequency and activation energy value of the process have been determined by means of full line-shape analysis. The rotational barrier increases with strengthening of the intramolecular hydrogen bond, as indicated by the δ OH chemical shift.

Direct 13C-31P coupling constant of coordinated triphenylphosphine as a characteristic of electron-withdrawing power of the metal center

It is known that C signals of phenyl groups attached to a phosphorus atom are split due to coupling with the P nucleus. The direct C–P coupling constant (JCP) sharply increases in going from aromatic phosphines to the corresponding phosphine oxides. In keeping with our and published data [1–10], the JCP value of PPh3 is negative and is –11 Hz, and the JCP value of Ph3P=O is positive (104 Hz). It is reasonable to rationalize increase of JCP by change of the valence state of the phosphorus atom. The phosphorus atom in the triphenylphosphine molecule possesses a lone electron pair (LEP), whereas the latter is involved in interaction with a strong electron acceptor (oxygen atom) in the phosphine oxide molecule.