Natural bond orbital

About: Natural bond orbital is a research topic. Over the lifetime, 7104 publications have been published within this topic receiving 147533 citations.

Natural hybrid orbitals

Abstract: From the information contained in the (exact or approximate) first-order density matrix, we describe a method for extracting a unique set of atomic hybrids and bond orbitals for a given molecule, thereby constructing its “Lewis structure” in an a priori manner. These natural hybrids are optimal in a certain sense, are efficiently computed, and seem to agree well with chemical intuition (as summarized, for example, in Bent’s Rule) and with hybrids obtained by other procedures. Using simple INDO-SCF-MO wave functions, we give applications of the natural hybrid orbital analysis to molecules exhibiting a variety of bonding features, including lone pairs, multiple bonds, strained rings, and “bent bonds”, multiple resonance structures, hydrogen bonds, and three-center bonds. Three examples are described in greater detail: (i) “orbital following” during ammonia umbrella inversion, (ii) the dimerization of water molecules, and (iii) the hydrogen-bridged bonds of diborane.

Natural bond orbital analysis of near‐Hartree–Fock water dimer

Abstract: We have carried out a natural bond orbital analysis of hydrogen bonding in the water dimer for the near‐Hartree–Fock wave function of Popkie, Kistenmacher, and Clementi, extending previous studies based on smaller basis sets and less realistic geometry. We find that interactions which may properly be described as ‘‘charge transfer’’ (particularly the n‐σ*OH interaction along the H‐bond axis) play a critical role in the formation of the hydrogen bond, and without these interactions the water dimer would be 3–5 kcal/mol repulsive at the observed equilibrium distance. We discuss this result in relationship to Klemperer’s general picture of the bonding in van der Waals molecules, and to previous theoretical analyses of hydrogen bonding by the method of Kitaura and Morokuma.

Natural localized molecular orbitals

Abstract: The method of natural localized molecular orbitals (NLMOs) is presented as a novel and efficient technique for obtaining LMOs for SCF and CI wave functions. It is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one‐particle density matrix. Results are presented for methane and cytosine to indicate that NLMOs closely resemble LMOs obtained by the Boys and Edmiston–Ruedenberg methods, with the exception that the NLMO procedure automatically preserves the MO σ–π separation in planar molecules. The computation time is modest, generally only a small fraction of the SCF computation time. In addition, the derivation of NLMOs from NBOs gives direct insight into the nature of the LMO ‘‘delocalization tails,’’ thus enhancing the role of LMOs as a bridge between chemical intuition and molecular wave functions.

Analysis of the geometry of the hydroxymethyl radical by the “different hybrids for different spins” natural bond orbital procedure

Abstract: We have carried out ab initio UHF/6-31G* calculations on the hydroxymethyl radical, CH 2 OH, and have found the equilibrium structure to be nearly planar with barriers to internal rotation occurring at staggered and eclipsed geometries, in good agreement with experiment. The electronic structure of the radical was analyzed via the “different hybrids for different spins” natural bond orbital (DHDS NBO) procedure, which finds separate Lewis structures for each of the spin systems. The α spin Lewis structure resembles that of the anion; the β spin Lewis structure resembles the corresponding cation. This simple picture, in conjunction with Bent's rule, allows one to understand the principal electronic factors which dictate the structure of the radical CH 2 group and its torsional and inversion potentials. Charge transfer between oxygen non-bonding orbitals and the empty radical orbital in the β spin system is the dominant interaction determining the torsional potential. Smaller hyperconjugative interactions in the α spin system resemble interactions in closed-shell molecules and directly oppose the effect of radical hyperconjugation, thus illustrating the central idea that open-shell potential energy features result from competition between the two different spin systems.