Ab initio calculations up to MP2/aug-cc-pVTZ//MP2/cc-pVTZ level, including natural charge population and natural resonance theory analyses, have been carried out to study the two-way effects between hydrogen bond (H-bond) and the intramolecular resonance effect by using the H-bonded complexes of formamide ( FAO) and its derivatives ( FAXs, X represents the heavy atoms in the substituent groups, CH 2, NH, SiH 2, PH, and S) with water as models. Unlike NH 3 and NH 2CH 3 which prefer being H-bond acceptors ( HA) to form H-bond with water, the amino groups in the six monomers, because of the resonance effect, prefer being H-bond donors ( HD) rather HA. Six monomers can all form HD complexes with water, and only two ( FAC and FASi) with the weakest resonance effect are able to form HA complexes with water. The HD H-bond and resonance effect enhance each other (positive two-way effects) whereas the HA H-bond and resonance effect weaken each other (negative two-way effects). The H-bond energies in the six HD complexes are nearly linearly correlated with the weights of the dipolar resonance in Pauling's model and the N-C bond lengths; the correlation coefficients are 0.91 and 0.93, respectively. The positive two-way effects also happens in FAO-water complex, in which the FAO CO group serves as HA ( HA co ). Interestingly, when the HD and HA co H-bonds are present in FAO H-bond complex simultaneously, the enhancements are much more significant, and the energies of the two types of H-bonds are much larger than those when only one type of H-bond is present, reflecting the cooperative effects. By using the knowledge to the two-way effects, we computationally designed a molecule ( FAO- BH 3 ) to increase H-bond energy. Because of the oxygen lone pair donation to the empty pi orbital of BH 3, FAO- BH 3 has a much stronger resonance effect than FAO. As a result, the H-bond energy (-5.55 kcal/mol) in HD H 2O ... FAO- BH 3 complex is much greater than the -3.30 kcal/mol in the HD H 2O...FAO complex. The two-way effects can be rationalized as follows: the resonance effect leads to intramolecular charge shifts in the monomers which facilitate or prevent the charge donation or acceptation of their H-bond partners. Therefore, the H-bonds are strengthened or weakened. In reverse, the charge donations or acceptations of their H-bond partners facilitate or prevent the intramolecular charge shifts in the monomer moieties, which enhance or weaken the resonance effect. The understanding to the two-way effects may be helpful in drug design and refinement by modulating the H-bond strength and in building empirical H-bond models to study large biological molecules. The study supports Pauling's resonance model.
For a theoretical model study on the cyclic reaction of 4-hydroxybutanal (4-OH-BL), we have examined five assumed reaction pathways (I–V) by performing the B3LYP calculations in the gas phase and self-consistent isodensity polarized continuum model (SCIPCM)-B3LYP calculations in aqueous solution. Pathways II (4-OH-BL + H+), III (4-OH-BL + H3O+), and IV (4-OH-BL + H3O+ + H2O) represent three models for the cyclic reaction catalyzed by Brønsted acids. The present study leads to the following conclusions concerning the five pathways (mainly on the basis of the calculation results in the solution). The high barrier along pathway I (with no catalyst) implies that the reaction does not occur without a catalyst, and the extremely large stabilization energy of the intermediate implies that pathway II is not a realistic model for the reaction catalyzed by Brønsted acid. Along pathway III, there are two intermediates and a transition state in between, and they are 10–16 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+). Along pathway IV, there is only one intermediate, and it is 20.6 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+ + H2O). Pathways III and IV are predicted to be feasible. Energetically, pathway IV is more favourable than pathway III and it is considered as a rational model for the cyclic reaction of 4-OH-BL catalyzed by Brønsted acid. Our calculations for pathway V (catalyzed by H2O) indicate that the water molecule may also serve as a catalyst for the cyclic reaction. The transition state along pathway V is 20.0 kcal/mol higher in energy than the reactants (4-OH-BL + H2O), and one can clearly see the “proton wire” in its structure. Our calculations show strong solvent effects on energetics of the charged intermediates along pathways II, III, and IV.
α decay Spin and parity Reduced α-decay width Quadrupole-octupole deformation Fine structure in the α decay of 223 U was observed in the fusion-evaporation reaction 187 Re( 40 Ar, p3n) by using fast digital pulse processing technique. Two α-decay branches of 223 U feeding the ground state and 244 keV excited state of 219 Th were identified by establishing the decay chain 223 U α1 −→ 219 Th α 2 −→ 215 Ra α 3−→ 211 Rn. The α-particle energy for the ground-state to ground-state transition of 223 U was determined to be 8993(17) keV, 213 keV higher than the previous value, the half-life was updated to be 62 +14 −10 µs. Evolution of nuclear structure for N = 131 even-Z isotones from Po to U was discussed in the frameworks of nuclear mass and reduced α-decay width, a weakening octupole deformation in the ground state of 223 U relative to its lighter isotones 219 Ra and 221 Th was suggested.
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