Abstract
All electrons SCF-LCAO-MO computations for adenine, cytosine, guanine and thyamine are reported. In addition, to compute the total energies and wave functions we have computed the relative gross charges and the dipole moment. Analysis of the orbital energy for the inner shell indicates that there are three effects which govern the orbital energies splitting for inner shell a) the gross charge, or ionicity degree, of the atom in consideration, b) its valency state, c) the neighbor atoms ionicities. The first two effects are sufficient for determining the relative location of the inner shell as well as for estimating the extrema of the splitting of the inner shell electrons of a given type of atoms.
The gross charge population was used to determine the overall flow of the σ and π charge transfer. It was found that the charge transfer flow requires direct consideration of at least next nearest neighbors to be explained. In addition, it was found that simple π electron considerations could lead to not only quantitative but even to qualitative erroneous prediction about the electronic charge distribution. For example, an atom can be positively charged, if one considers only the π electrons, and the same atom can be negatively charged, if one considers only the σ electrons. Therefore, we reiterate on the necessity of all electron computations not only for quantitative but even for qualitative studies of the electronic structure in molecules.
Резюме
Излагаются все электроны, определенные методом SCF-LCAO-MO, для аденина, цистозина, гуанина и тиамина. С целью вычисления полной энергии и волновых функций определены относительно большие заряды и дипольный момент. Анализ орбитальной энергии для внутренних оболочек показывает, что имеется три эффекта, которые оказывают влияние на орбитальные энергии по отношению внутренних оболочек а) полный заряд, или степень ионизации исследуемого атома; б) его валентное состояние; в) ионизации соседних атомов. Первые два эффекта достаточны для определения относительного расположения внутренней оболочки, для оценки экстремума расщепления электронов внутренних оболочек атомов данного типа.
Полная зарядная населенность использовалась для определения универсального потока переноса заряда δ и π. Найдено, что поток переноса заряда требует непосредственного рассмотрения по крайней мере следующих самых близких соседей. Наконец, показывается что рассмотрения просто π-электронов могут проводить не только к качественному, но и к количественному ошибочному предсказанию о распределении электронного заряда. Например, атом может быть заряжен положительно, если рассматриваются только — электроны, и тот же атом может иметь отрицательный заряд, если рассматриваются лишь δ-электроны. На основе этого мы подчеркиваем необходимость принятия во внимание всех электронов при вычислениях не только с целью количественного, но даже и качественного изучения электронной структуры молекулы.
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E. Clementi andD. Davis, J. Comp. Phys.,1, 223, 1966.
E. Clementi, J. Chem. Phys.,46, 3842, 1967; 3851, 1967; 4725, 1967; 4731, 1967; 4737, 1967; ibid.47, 2323, 1967; 3837, 1967; 4485, 1967.
E. Clementi, V. Coiro andE. Rusconi, J. Chem. Phys., Study in the Electronic Structure of Molecules. XII, Inner Shells and Aufbau Principle (to be published).
Somewhat parenthetically we would like to note that the ∼n 4 number (more accurately (N 2+N)/2, whereN=(n 2+n)/2 ifn is the basis set size) is most definitely only an upper limit. The larger the molecule (say 5 to 6 first row atom and up), the more the above statement is true since a large (up to 50 to 90%) number of integrals is exceedingly small (of the order of equal or less than 10−8 atomic units).
Appendix to IBM Technical Report: Study in the Electronic Structure of Molecules. X. Ground State Functions for adenine, cytosine, guanine and thyamine.
The gross population analysis is computed according toR. S. Mulliken, J. Chem. Phys.,23, 1833, 1841, 2243, 1955. See in addition the first paper in this series (E. Clementi, J. Chem. Phys.,46, 3842, 1967).
The atomic unit for energy corresponds to 27.2098 e.v. for length to 0.5292 Å, for dipole moments to 2.5416 debyes.
E. Clementi, Tables of Atomic Functions. Supplement to a paper byE. Clementi, IBM Journal of Res. and Dev.,9, 2, 1965.
Let us now take into consideration the neighbors effect. This effect has been taken into consideration indirectly, by using the ionic charges. Clearly, the charge transfer takes place because there are neighbors. However, we wish to take into account the nearest neighbors more directly. In other words, the fieldF of the equationFΦ i =ε i Φ i is the field of the atom (after loss or gain of charges because of charge transfer) plus the field of the neighboring atoms, i.e.,F=F l+FN, whereF l is the local (nearly atomic field) andF N the neighbors field. This will affect thei which can be obtained by perturbation of theε ic whereε il is the orbital energy of the separated atoms, with correct ionic character and in the valency state. A positive charge in the neighborhood of an atom will affect its inner shell by polarizing the inner shell electrons toward that charge and will increase (absolute value) its energy. A negative charge in the neighborhood of an atom will affect its inner shell by polarizing the inner shell electrons away from that charge and will likely affect its energy less than for positive charges. A systematic study of point charge variation on the orbital energies restricted however, to CN-molecules, seems to support this point [10]. The inner shell variation due to presence of point charges (positive or negative) in the CN− system is of the order of few hundreds of an atomic unit or sufficient to bring the data of column three in better agreement with those of column one (in Table XX). At this stage we can only content ourselves with having established a reason for the magnitude of the splitting and in having correlated correctly the relative position of the computed orbital energies with our ionic model. Point charge perturbations are in progress to elucidate the neighboring charges effect.
E. Clementi andD. Klint, Study in the Electronic Structure of Molecules, IX. The Cyano Group (J. Chem. Phys. in press).
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Dedicated to Prof.P. Gombás on his 60th birthday.
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Clementi, E., André, J.M., André, M.C. et al. Study of the electronic structure of molecules. X. Acta Physica 27, 493–521 (1969). https://doi.org/10.1007/BF03156769
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DOI: https://doi.org/10.1007/BF03156769