Resonance structures dominant

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

When different resonance structures are possible, some giving the central atom in a compound an octet and some an expanded valence shell, the dominant resonance structure is likely to be the one with the lowest formal charges. However, there are many exceptions and the selection of the best structure often depends on a careful analysis of experimental data. 

EXAMPLE 2.8 Selecting the dominant resonance structure for a molecule... 

Octet expansion (expansion of the valence shell to more than eight electrons) can occur in elements of Period 3 and later periods. These elements can exhibit variable covalence and be hypervalent. Formal charge helps to identify the dominant resonance structure. 

The second of the resonance structures is the source of the radical reactivity displayed during oxidation and the Mn(III) in this structure must be low-spin to preserve multiplicity. Substitution at the meso-position could provide steric hindrance to analogous decompositions and reactions of the Mn(IV) complexes of ethyl- and benzylmalonic acids, and a conventional one-equivalent oxidation step becomes dominant. 

The methyl group responds to the difference in the three-dimensional electron density distribution about the two nearest ring CC bonds, and the natural bond orders most simply quantifies the key difference in a unified manner across many molecules. At one extreme, 2-methylpropene has essentially localized single and double bonds (03-0b = 1) and a 1010 cm-1 barrier. At the other extreme, when the geometry of the ring has good local C2v symmetry, as in the S0 state of toluene, m-fluorotoluene, p-fluorotoluene, 3,5-difluorotoluene, and 2,6-difluorotoluene, 03 - Ob and the barrier is invariably very small, even for nominal threefold cases. We interpret this equality of bond orders as indicative of essentially equal contributions of the two dominant resonance structures at all a. 

A variety of acyclic and cyclic S-N compounds decompose at moderate temperatures (100-150 °C) with the formal loss of a symmetrical NSN fragment, but this molecule has never been detected. The lowest energy isomer, linear NNS, is generated by flash vacuum pyrolysis of 5-phenyl-l,2,3,4-thia-triazole.40 Ab initio molecular orbital calculations indicate that the resonance structure N = N+-S is dominant.41... 

In practice, a simplified single-reference form of Eq. (1.38) is used for the case of a weakly delocalized dominant resonance structure with u) w2, w3,... see note 53. 

Because trans dispositions commonly result from co-bonding (the near-linear alignment of the hyperbonding 3c/4e X—M- L triad), it is not surprising that the origin of the trans influence can be traced to the resonance nature of co-bonding. When H is placed trans to a halide or PH3, the dominant resonance structure will be that with a 2c/2e M—H bond and a donor pair of electrons on the halide or phosphine ligand, as depicted on the left in (4.93) ... 

The ring torsional angles of the amide and O-protonated amide moieties in these structures are close to zero and the C(O)—N bond length is shorter in the protonated lactam (445) (1.298 A) than it is in the lactam itself (444) (1.337 A), as expected from the dominant resonance structures in these two compounds. The room temperature 15N and 13C NMR spectra of the lactam (444) and its O-trimethylsilyl derivative have been determined but do not give much information about the solution conformation of these compounds (76JA5082, 82JMR(46)163>. 

The stabilized ylides have at least one substituent on the ylidic carbon that can either conjugate with the P=C rcbond (16a) or delocalize the carbanion charge (16c). Most of the crystal structures of the stabilized ylides have a carbonyl group attached to the ylidic carbon, although other groups are represented. The dominant resonance structures of the carbonyl stabilized ylides are shown in 16a-c. A summary of the X-ray structural data for the stabilized ylides is given in Table 3. 

In conclusion, the dominant resonance structures for the ylides are as follows for non-stabilized ylides, A for stabilized ylides, 16b and 16c for P=C=P ylides, 22 (although further study on this class is advised to determine if the gas-phase structure may in fact be linear) and for P=C=C ylides, 28b and 28c. 

Wang and Schleyer have investigated various 1,1-disubstituted-phosphorins 9 and arsenins 10 at the B3LYP/6-31G DFT level <2001HCA1578>. Based on the NICS(l) values, they find that the compounds are nonaromatic ylides (resonance structures 9 and 10 dominant) for electropositive substituents. However, for electronegative substituents, the compounds are aromatic. 

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