Brauer Brina, Gerber R Benny, Kabelác Martin, Hobza Pavel, Bakker Joost M, Abo Riziq Ali G, de Vries Mattanjah S
Department of Physical Chemistry and Fritz Haber Research Center, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
J Phys Chem A. 2005 Aug 11;109(31):6974-84. doi: 10.1021/jp051767m.
The results of harmonic and anharmonic frequency calculations on a guanine-cytosine complex with an enolic structure (a tautomeric form with cytosine in the enol form and with a hydrogen at the 7-position on guanine) are presented and compared to gas-phase IR-UV double resonance spectral data. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces; the improved PM3 potential surfaces are obtained from standard PM3 theory by coordinate scaling such that the improved PM3 harmonic frequencies are the same as those computed at the RI-MP2/cc-pVDZ level. Comparison of the data with experimental results indicates that the average absolute percentage deviation for the methods is 2.6% for harmonic RI-MP2/cc-pVDZ (3.0% with the inclusion of a 0.956 scaling factor that compensates for anharmonicity), 2.5% for harmonic RI-MP2/TZVPP (2.9% with a 0.956 anharmonicity factor included), and 2.3% for adapted PM3 CC-VSCF; the empirical scaling factor for the ab initio harmonic calculations improves the stretching frequencies but decreases the accuracy of the other mode frequencies. The agreement with experiment supports the adequacy of the improved PM3 potentials for describing the anharmonic force field of the G...C base pair in the spectroscopically probed region. These results may be useful for the prediction of the pathways of vibrational energy flow upon excitation of this system. The anharmonic calculations indicate that anharmonicity along single mode coordinates can be significant for simple stretching modes. For several other cases, coupling between different vibrational modes provides the main contribution to anharmonicity. Examples of strongly anharmonically coupled modes are the symmetric stretch and group torsion of the hydrogen-bonded NH2 group on guanine, the OH stretch and torsion of the enol group on cytosine, and the NH stretch and NH out-of-plane bend of the non-hydrogen-bonded NH group on guanine.
本文给出了对具有烯醇结构的鸟嘌呤 - 胞嘧啶复合物（一种互变异构形式，胞嘧啶为烯醇形式，鸟嘌呤7位有一个氢原子）进行的谐波和非谐波频率计算结果，并与气相红外 - 紫外双共振光谱数据进行了比较。谐波频率是在RI - MP2/cc - pVDZ、RI - MP2/TZVPP和半经验PM3电子结构理论水平上获得的。非谐波频率是通过CC - VSCF方法结合改进的PM3势能面得到的；改进的PM3势能面是通过坐标缩放从标准PM3理论获得的，使得改进后的PM3谐波频率与在RI - MP2/cc - pVDZ水平计算的频率相同。数据与实验结果的比较表明，对于谐波计算，RI - MP2/cc - pVDZ方法的平均绝对百分比偏差为2.6%（包含补偿非谐性的0.956缩放因子时为3.0%），RI - MP2/TZVPP方法为2.5%（包含0.956非谐性因子时为2.9%），改进的PM3 CC - VSCF方法为2.3%；从头算谐波计算的经验缩放因子提高了伸缩频率，但降低了其他模式频率的准确性。与实验的一致性支持了改进后的PM3势能在描述光谱探测区域中G...C碱基对非谐力场方面的充分性。这些结果可能有助于预测该系统激发后振动能量流动的途径。非谐波计算表明，对于简单的伸缩模式，沿单模坐标的非谐性可能很显著。在其他几种情况下，不同振动模式之间的耦合对非谐性起主要作用。强非谐耦合模式的例子包括鸟嘌呤上氢键连接的NH₂基团的对称伸缩和基团扭转、胞嘧啶上烯醇基团的OH伸缩和扭转，以及鸟嘌呤上非氢键连接的NH基团的NH伸缩和NH面外弯曲。
