Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and .sigma.-aromaticity

Abstract

V vs. N H E (turnover frequency-lo3 mol of C O produced per mol of nickel complex, in 1 h) and selective, even in a purely aqueous medium. The stability of the complex used makes it a promising electrocatalyst (I O4 electrocatalytic cycles on Ni cy-clam2+ without degradation). In the present study, we have detected a nickel(1) carbonyl complex which may participate in the catalytic cycle. Other important reaction parameters have been investigated. The efficiency of Ni cyclam2+ in the electroreduction of C02 may be due to the size of the cyclam ring (1 4 atoms), which greatly stabilizes nickel complexes. A reason for the selectivity of the electrocatalyst may be difunctional activation of C02. The acidic character of the N-H protons of cyclam could favor C 0 2 fixation by hydrogen bonding (N-H-0) in addition to the carbon to nickel(1) link.

Aromaticity of Tri‐ and Tetranuclear Metal–Carbonyl Clusters Based on Magnetic Criteria

“…[34] Results and Discussion 12 ] has a C 2v structure with two bridging groups, [5] in contrast to the D 3h symmetry of the rutheniumand osmium-carbonyl clusters with only terminal CO groups. [35] The stability of [M 3 (CO) 12 ] (M = Fe, Ru, Os) clusters is generally explained in terms of s-aromaticity, [12] which has long been invoked in the description of the electronic structure of cyclopropane; [11,36,37] strong evidence [11g] exists for this phenomenon. [15,[38][39][40][41][42][43] The concept was rationalized, for instance, in terms of the ellipticity of the density at its critical points.…”

“…In 1986, Gauss and Cremer [37] first described tetrahedrane and other cages formed by three-membered ring (3-MR) subunits as s-aromatic by using the notion of "volume delocalization". The diatropic nature of 3-MR-containing hydrocarbon and inorganic cages was then investigated in detail by Schleyer et al [13,15] Based on a NICS analysis, they identified tetrahedrane as being "super s-aromatic" and T d As 4 [13] as highly aromatic.…”

Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles – An Interpretation by Triadic Analysis

“…Cyclopropane is the paradigmatic strained carbocycle, having ring strain energy E strain = 27.5 kcal mol -1 , 11 and it has played a unique role in the study of ring strain, being a reference molecule to which other strained systems are typically compared. 12−14 In cyclopropane, the concept of ring strain is supported by the high electron density in the interior of the three-member ring, 15−17 the relatively short C-C and C-H bond distances, 15,16 and the upfield shifts of its proton 18 and 13 C NMR signals. 19 In most cases, ring strain energy cannot be determined experimentally; it is relative rather than absolute quantity that must be defined and assessed by comparison with arbitrary reference species considered by convention to be "strain free".…”

The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations

“…Stabilization by s-aromaticity is responsible for the abnormal behavior of cyclopropane. [76,83,89,90] HF, B3LYP, and MP2 computations with the 6-311++G-A C H T U N G T R E N N U N G (d,p) basis set show all three methods perform well for cyclopropane (Table 3). The nearly equal energy computed by these three methods leads us to conclude that protobranching in cyclopropane is small or nonexistent.…”

“…[83] Comparing the Baeyer angle strain of planar silicon and planar carbon rings (Figures 2 and 3, respectively) shows the unusual nature of cyclopropane. As Baeyer strain is only concerned with deviations from the idealized sp 3 -hybridized bond angle, one expects "V-shaped" or parabolic curves when strain energies are plotted against the size of planar rings.…”