Proton affinities calculation

Table 3 presents the proton affinities calculated (without zero-point and thermodynamic contributions) for both N- and 0-ends of N2O. It is seen that indeed the SCF result leads... 

FIGURE 22. Linear correlation between proton affinities calculated at the AMI semiempirical level and at the MP2/6-31+G(d,p)//6-31G level. (The numbers in this Figure refer to the compound numbers in Table 28)... 

The trends in metal ion-ethylene binding energies in Table 5 can be analysed as functions of basis set, theoretical level and metal ion. In contrast to the proton affinity calculation described above where zero-point energy difference contributed more than 6 kcalmoP to the final binding energy (BE) value, the vibrational frequencies in Table 7 show that the electronic energy part of the complex coordination energy will be reduced only by about... 

Kennedy RA, Mayhew QiA, Thomas R, Watts P (2003) Int J Mass Spectrom 223 627-637 Labanowskiy JK, Hill RA, Heisterbergy DJ, Miller DD, Bender CF, Andzelm JW (2011) Proton affinities calculated by traditional ab initio approaches and by density functional methods. http //www.ccl.net/cca/documents/proton-affinity/affinities.pdf. Accessed 2011 Lias SG, Liebman JF, Levine RD (1984) J Phys Chem Ref Data 13 695-808 Lias SG, Bartmess JE, Liebman JF, Holmes JL, Levin RD, Mallard WG (1988) J Phys Chem Ref Data 17 1-861... 

Thus, the values calculated for effective polarizability at the nitrogen atom for a series of 49 amines carrying only alkyl groups was correlated directly with their proton affinities, a reaction that introduces a positive charge on the nitrogen atom by protonation (Figure 7-7) [40. ... 

Figure 7-7. Equations for the calculation of proton affinities (PA) of simple alkyl amines and of heteroatom-substituted alkyl amines.

Data on proton affinities (gasphase) ofmany different compounds (see Table 2) demonstrate the high level of accuracy possible in determining energies of related species. In this report by Dewar and Dieter, the enthalpy of formation of is the experimental value (367.2 kcal/mol). The calculated value for is unreliable. 

Table 8—2. Proton affinities of selected compounds, from AMI calculations...

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

The original paper defining the Gaussian-2 method by Curtiss, Raghavachari, Trucks and Pople tested the method s effectiveness by comparing its results to experimental thermochemical data for a set of 125 calculations 55 atomization energies, 38 ionization potentials, 25 electron affinities and 7 proton affinities. All compounds included only first and second-row heavy atoms. The specific calculations chosen were selected because of the availability of high accuracy experimental values for these thermochemical quantities. 

The calculation of the proton affinities (PA) for a pair of tautomers and the comparison with experimental data [generally from ICR measurements (Section VII,F)] has been the subject of a series of publications with increasing sophistication (Table IV). Such calculations concerning the annular tautomerism of azoles and benzazoles have been reviewed [87AHC(41)187]. 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

Predicted proton affinities of azoles (and oxazoles) calculated with simple ab initio methods (STO-3G) are reported to differ little from 6-3IG values (89KGS508). 

MO studies (AMI and AMI-SMI) on the tautomerism and protonation of 2-thiopurine have been reported [95THE(334)223]. Heats of formation and relative energies have been calculated for the nine tautomeric forms in the gas phase. Tire proton affinities were determined for the most stable tautomers 8a-8d. Tire pyrimidine ring in the thiones 8a and 8b has shown a greater proton affinity in comparison with the imidazole ring, or with the other tautomers. In solution, the thione tautomers are claimed to be more stabilized by solvent effects than the thiol forms, and the 3H,1H tautomer 8b is the most stable. So far, no additional experimental data or ab initio calculations have been reported to confirm these conclusions. 

Calculated Proton Affinities and pK Values for Imidazo [4,5-/ Quinolines... 

Proton affinities of the bases calculated using the value AT/f (H ) = 367.2 kcal mol ... 

The structures of pyrrolo[l,2-c]pyrimidine 139 and its N-protonated form 140 were obtained from MP2/6-31G calculations (Scheme 92) [99JOC7788]. Proton affinities computed at the same level reveal that N-protonation is slightly preferred over protonation at the C7 position. The most stable tautomers of 2-substituted 5-methyl-7-hydroxy-l,2,4-tiiazolo[l,5-a]pyrimidine 141 were... 

Proton affinities of ethene (684 121) and 680129) kJ mol-1) measured experimentally correspond with results from ab initio calculations (698 kJ mol-1 130)). MINDO/3 calculations (with AHf(H+) = 1528 kJ mol-1 91)) also deliver a result of comparable value (714 kJ mol 1) when the formation of a classical carbocation during the protonation is assumed. 

In the present work, we shall investigate the problem of the amount of correlation accounted for in the DF formalism by comparing the molecular electrostatic potentials (MEPs) and dipole moments of CO and N2O calculated by DF and ab initio methods. It is indeed well known that the calculated dipole moment rf these compounds is critically dependent on the level of theory implemented and, in particular, that introduction of correlation is essential for an accurate prediction [13,14]. As the MEP property reflects reliably the partial charges distribution on the atoms of the molecule, it is expected that the MEP will exhibit a similar dependence and that its gross features correlate with the changes in the value of dipole moment when switching from one level of theory to the other. Such a behavior has indeed been reported recently by Luque et al. [15], but their study is limited to the ab initio method and we found it worthwhile to extend it to the DF formalism. Finally, the proton affinity and the site of protonation of N2O, as calculated by both DF and ab initio methods, will be reported. 

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