Hydrogen bond. Environmental effects on proton potential curves. An SCRF MO CNDO/2 calculation of a water dimer

A solvent and/or environmental effect has been introduced into the MO CNDO/2 calculation of a model hydrogen bonded system. Proton potential curves, potential energy, dipole moments, the polarizability component parallel to the reaction field, and the second order perturbation effects associated to the dispersion forces, have been studied as a function of a solute-solvent coupling parameter. This parameter may be related, through the self-consistent reaction field theory of solvent effects, to both the macroscopic dielectric properties of the solvent and to the local order (if any) around the solute. Numerical results corresponding to a water dimer are discussed.     

“Universal” CNDO/2 method (CNDO/2-U)

The Pople-Santry-Segal CNDO/2 method and the modified CNDO/2 version worked out recently by Clack for transition-metal complexes have been extended to the full periodic table.     

High dilution measurement of the chemical shift of phenolic protons: Correlations with the electronic charge calculated by CNDO/2

The chemical shifts of the hydroxylic protons in a series of substituted phenols have been measured in highly-diluted CCl₄ solutions. The linear correlations between chemical shifts and the charges on the hydroxylic proton and the oxygen atom, calculated by CNDO/2 method indicate that a 0.013 and 0.008 electron variation of the charge on the hydrogen and oxygen atom, respectively, involve a 1 ppm variation of the chemical shift.     

Calculations for para-substituted phenols using the CNDO/2 method

Theoretical and Experimental Chemistry -     

Assignments in photoelectron spectroscopy using the CNDO/2 method

In the photoelectron spectra of molecules vibrational fine structure often accompanies the observed ionization potentials (IPs). For non-diatomic molecules it is difficult to make vibrational assignments because the frequencies of the ion often differ greatly from those of the neutral molecule. In order to make both the vibrational assignments and the assignments of the IPs to molecular orbitals (MOs) using Koopmans' theorem CNDO/2 MO calculations have been carried out. Within the framework of this method a quantity E _AB has been calculated which is the change in potential energy between an atom-pair A-B for a particular MO on ionization. Application to some diatomic molecules has shown that this quantity reflects the changes in vibrational frequencies on ionization. The IP-MO assignments are made such that removal of an electron from a particular MO is in accord with the observed vibrational fine-structure; attempting to make these assignments on the basis of the calculated MO ordering alone has been shown to be very unsatisfactory.

---

Zusammenfassung Photoelektronenspektren von Molekülen zeigen neben den Ionisierungspotentialen oft noch eine Schwingungsfeinstruktur, deren Zuordnung bei nicht-zweiatomigen Molekülen wegen der starken Frequenzverschiebungen vom Molekül zum Ion Schwierigkeiten macht. Um sowohl Schwingungsais auch Ionisierungs-Zuordnungen vornehmen zu können, wurden Rechnungen mit dem CNDO/2-Verfahren durchgeführt. Dabei stützt man sich auf die Änderung der potentiellen Energie zwischen dem Atompaar AB in bezug auf ein bestimmtes MO bei Ionisierung (E _AB). Die Anwendung auf zweiatomige Moleküle zeigt, daß diese Größe tatsächlich die Änderung der Schwingungsfrequenzen wiedergibt, und die Zuweisungen des IP werden so vorgenommen, daß die Entfernung eines Elektrons aus einem MO im Einklang mit der Schwingungsfeinstruktur ist. Dagegen erweist sich die einfache Zuweisung auf Grund der MO-Reihenfolge als schlechter.

     

A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

---

Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

---

Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

---

---

This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

CNDO/2法计算二茂锆卤化物的电子结构

本文将Pople的闭壳层CNDO/2程序改编成将原子中s,p,d亚层分开处理,并扩展到可计算锆化合物的CNDO/2-TM程序;选取了锆的有关参数;计算了Cp_2ZrX_2(X=F,Cl)分子的电子结构.得到的氯化物分子轨道的能级次序和间隔与光电子能谱相符.计算的偶极矩为5.88D,与实验值相近.计算结果还说明Cp_2ZrX_2是有一定离子性的共价化合物.中心锆原子约呈 1价.金属锆的5s轨道参与茂环的σ键,5p轨道通过移入电子参与茂环σ键,而4d轨道则参与茂环的π键,但成键所占成分不大.Zr-F键的离子性占51％,Zr-Cl键的离子性占44％,Zr-Cp键的共价性在84～93％之间.     

CNDO/2 calculations of the interactions of formamide and imidazole: Another failure of CNDO/2

The interaction energy curve of the formamide-imidazole system resulting from the CNDO/2 method exhibits a superfluous minimum at a much smaller separation than the sum of the van der Waals radii of the closest atoms, and a low maximum at this latter distance. The relationship of the interaction energies using CNDO/2 and those using the atomic charges on the two molecules is investigated for this system.     

Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

CNDO calculation of intermolecular V-T and V-V potentials. The CO2-N2 system

The intramolecular electron transfer between two naphthyl moieties (N) of trans-1,4-bis-(α-naphthyl-methyl)-cyclohexane in HMPA was studied over a range of temperatures

. Although the inspection of the model indicates a separation by 9 Å between the naphthyl groups the frequency of transfer is 0.9 × 10⁷ s^?1 at 15°C.