Application of Schrödinger Equation to Study the Tunnelling Dynamics of Proton Transfer in the Hydrogen Bond of 2,5-Dinitrobenzoic Acid: Proton T1, T1g

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• 作者：L. Latanowicz ; W. Medycki
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：February 22, 2007
• 年：2007
• 卷：111
• 期：7
• 页码：1351 - 1357
• 全文大小：105K
• 年卷期：v.111,no.7(February 22, 2007)
• ISSN：1520-5215

文摘

Temperature measurements of proton T₁ (24.7 MHz), deuteron (deuterated hydroxyl group) T₁ (55.2 MHz),and proton T_{1ges/gifchars/rho.gif" BORDER=0 >} (B₁ = 9 G) spin-lattice relaxation times of 2,5-dinitrobenzoic acid have been performed. Ananalysis of present experimental data together with previously published proton T₁ (55.2 MHz) data has revealedthe following molecular motions: proton/deuteron transfer in the hydrogen bond and two-site hopping of thewhole dimer. It is shown that the proton-transfer dynamics are characterized by two correlation times ges/gifchars/tau.gif" BORDER=0 >^ov andges/gifchars/tau.gif" BORDER=0 >^tu, describing two fundamentally different motional processes, namely, thermally activated jumps over thebarrier and tunneling through the barrier. The temperature dependence of 1/ges/gifchars/tau.gif" BORDER=0 >^tu is the solution of Schrödinger'sequation, which also yields the temperature T_tun, where begins the tunnel pathway for proton transfer. A newequation for the spectral density function of complex motion consisting of the three motions is derived. Thethird motion (two-site hopping of the whole dimer characterized by ges/gifchars/tau.gif" BORDER=0 >^lib correlation time) is responsible for aproton T_{1ges/gifchars/rho.gif" BORDER=0 >} minimum in high temperatures, just below the melting point. Such a minimum is not reached byT₁ temperature dependencies. The minimum of T_{1ges/gifchars/rho.gif" BORDER=0 >} assigned to the classical hopping of a hydrogen-bondedproton occurs in the same low-temperature regime in which the flattening of the temperature dependenciesof T₁ points to the dominance of incoherent tunneling. This experimental fact denies the known theoriespredicting the intermediate temperature regime where a smooth transition between classical and quantumtunneling dynamics is expected. The fit of the derived theoretical equations to the experimental data T_{1ges/gifchars/rho.gif" BORDER=0 >} andT₁ is satisfactory. The correlation times obtained for deuterons indicate deuteron-transfer dynamics muchslower than proton-transfer dynamics. It is concluded that the classical proton transfer takes place over thewhole temperature regime, while the incoherent tunneling occurs below 46.5 (hydrogen) or 87.2 K (deuterium)only.