Proton affinity formaldehyde

Thus, the three-center potential integrals are not retained. Ferenczy et al. found that the AMI MEP maps are able to reproduce the main characteristics of the HF/STO-3G MEP maps for the water, formaldehyde, formamide and the cytosine molecules and for the cyanate ion [40]. However, the NDDO AMI MEP maps gave deeper minima which were closer to the molecules than those in the HF/STO-3G MEP maps. The contour plots of MEP for the cytosine molecule are displayed in Fig. 2. It can be seen that the NDDO AMI MEP map correctly predicts the N3 nitrogen atom as a primary protonation center instead of the 02 oxygen atom. This finding is in agreement with the HF/STO-3G MEP map [28] and the experimental as well as the theoretical proton affinities [29, 30]. Similar results were also obtained by Luque et al. based on the quasi ab initio MNDO MEP map [37], INDO/S, HF/4-31G and HF/6-31G calculations showed an opposite order of protonation [27, 31] as discussed earlier. 

In order to protect the proton, and thereby suppress the kinetically favoured proton transfer route, it has been found out that gas-phase addition followed by elimination can be enhanced by reacting the proton bound dimer of the carbonyl compound rather than the protonated monomer [ 134]. In cases where the carbonyl compound has a higher proton affinity than the nucleophile, proton transfer is of course no problem. Alternatively, if the nucleophile already is protonated, as in the reactions between NH] and various carbonyl compounds, proton catalysed addition/elimination is possible as demonstrated experimentally by observation of immonium ion formation [135-137]. Likewise, the hydrazo-nium ion has been found to react with formaldehyde and a wide range of other aldehydes and ketones [138]. 

Proton affinity (PA) acetaldehyde, 123 acetone, 123 acrolein, 123 butenone, 123 dimethyl ether, 123 dimethylacrolein, 123 formaldehyde, 123 methanol, 123 methyl acetate, 123 methyl acrylate, 123 (Aj-mcthylacrolein, 123 oxetane, 123 table of, 123 tetrahydrofuran, 123 water, 123... 

Arnold et al. (1977) proposed that the non-proton hydrates are formed by reactions of proton hydrates with formaldehyde, which possesses a large proton affinity, but this hypothesis was rejected when the reaction of formaldehyde with higher order proton hydrates was found to be endothermic (Fehsenfeld et al., 1978). Ferguson (1978) suggested that proton hydrates might react rapidly with NaOH. If sodium (injected near 90 km by meteor ablation) could reach the stratosphere, it should be present there as NaOH, a species with a large proton affinity. 

Many carbonyl addition and substitution reactions are carried out under acidic conditions or in the presence of Lewis acids. Qualitatively, protonation or complexation increases the electrophilicity of the carbonyl group. The structural effects of protonation have been examined for formaldehyde, acetaldehyde, acetone, formamide, and formyl fluoride. These effects should correspond to those in more complex carbonyl compounds. Protonation results in a substantial lengthening of the C=0 bond. The calculated [B3LYP/ 6-31-H-G(phase proton affinities reflect the trend of increasing basicity with donor groups (CH3, NH2) and decreased basicity for fluorine. 

Problems of detection also arise for compounds whose proton affinities are close to that of water (PA(H20) = 691 kJ mor ). This occurs for several compounds that may be significant in an atmospheric environment, including hydrogen sulfide (PA(H2S) = 705 kJ mor ), hydrogen cyanide (PA(HCN) = 713 kJ moP ) and formaldehyde (PA(HCHO) = 712 kJ mol ) (the PAs for the above molecules have been taken from reference [46]). (Generally, inorganic molecules have proton affinities below that of water but H2S and HCN are clear exceptions to this rule.) The closeness of the proton affinities of H2S, HCN... 

The first cucurbituril derivative decamethylcucurbit[5]uril (Me-CB[5]) (Chart I) was synthesized from dimethylgly-coluril and formaldehyde in 1992. " The X-ray structural analysis shows a cavity of 4 A in diameter and two identical portals of 2.5 A in diameter. Bradshaw and coworkers studied complexation of Me-CB[5] with metal ions in 50% formic acid/water solution by calorimetric and potentiometric methods.Me-CB[5] binds most metal ions in a 1 1 stoichiometry in the acidic solution. Interestingly, Me-CB[5] shows exceptionally high affinity for Pb ion (log K > 9). which is mainly due to the size match between Pb and Me-CB[5] portals. The p a value (9.56) for protonation of Me-CB[5] indicates that Me-CB [5] behaves as a base in an aqueous solution. 

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