Mendeleev Communications Electronic Version, Issue 4, 1998 (pp. 129–168) Ab initio study of the structure of, and double proton exchange in, 1,4-dihydroxy-2,3-diformylbuta-1,3-diene Tatyana N. Gribanova, Ruslan M. Minyaev* and Vladimir I. Minkin Institute of Physical and Organic Chemistry, Rostov State University, 344090 Rostov-on-Don, Russian Federation. Fax: +7 8632 28 5667; e-mail: minyaev@ipoc.rnd.runnet.ru Concerted low-energy barrier (3.7 kcal mol–1) double proton exchange in 1,4-dihydroxy-2,3-diformylbuta-1,3-diene has been predicted using ab initio [MP2(fc)/6-31G**] calculations.Particular attention has been given to the study of the kinetics and mechanism of the intramolecular two proton migration in oxalic acid,1 oxalamidine,2 azophenine,3,4 2,2'-bipyridyl- 3,3'-diole5,6 and other similar compounds.7,8 Both theoretical and experimental investigations1–8 showed that all the dyotropic rearrangements studied follow a two-step mechanism involving sequential proton transfer with inclusion of a zwitterionic intermediate.No unambiguous experimental or theoretical evidence for the realization of the concerted (one-step) double proton transfer within a molecule have hitherto been presented.In the present work we report on ab initio [MP2(fc)/6-31G**]9 calculations of a concerted low-energy barrier (3.7 kcal mol–1) degenerate rearrangement of 1,4-dihydroxy-2,3-diformylbuta- 1,3-diene 1 due to intramolecular double proton transfer. According to the MP2(fc)/6-31G** calculations, a planar structure 1 (l = 0; hereafter l designates the number of negative eigenvalues at a given stationary point) with C2h-symmetry corresponds to the most stable form of the 1,4-dihydroxy- 2,3-diformylbuta-1,3-diene. A possible cis-(Z)-conformer 3 is 2.6 kcal mol–1 less favourable than 1.Unlike 1, the isomer 3 is acoplanar (C2-symmetry) with the dihedral C=C–C=C angle equal to 61.6°. Calculated molecular structures, geometry and energy parameters of the structures 1–3 are given in Figure 1 and Table 1.The symmetric structure 2 of D2h-symmetry corresponds to a true saddle point (l = 1) on the potential energy surface (PES) of C6H6O4. A possible zwitterionic intermediate 4 that would result from single-proton transfer does not correspond to a stationary point. Optimizations starting from the zwitterionic configuration 4 with C2v and C1 symmetries lead to structures 2 and 1, respectively.Thus, there exists only the concerted proton exchange pathway 1a 2 1b in 1,4-dihydroxy- 2,3-diformylbuta-1,3-diene which implies occurrence of the multicentered transition state structure 2 with a very low energy of 3.7 kcal mol–1 relative to 1. The three-centre hydrogen bridges in 2 are nearly linear (deviation from linearity is ca. 12°). The H···O=C angle of 112.6° lies within the limits of the optimal values for proton transfer along the hydrogen bond.10 Accounting for zero-point energy corrections in 1 and 2 leads to the conclusion that the bicyclic structure 2 with hydrogen atoms centered in the middle of the O···O bridge possesses lower total energy as compared with 1.A similar phenomenon of the vibrational stabilisation of the structure with symmetrical hydrogen bridges has been discussed11 recently with reference to experimental data for the IHI system.12 Thus, our calculations corroborate the assumption about the crucial influence of the stereochemical conditions on the proton transfer mechanism. Structure 1,4-dihydroxy-2,3-diformylbuta- 1,3-diene appears to be the first example of a dyotropic molecule in which one-step low-barrier double proton exchange confirmed at the MP2-level is possible.This work was supported by the Russian Foundation for Basic Research (grant nos. 98-03-33169a and 96-15-97476). O O O O H H H H H H O O O O H H H H H H O O O O H H H H H H 1a 2 1b O O H O H O O OH OH O 3 4 1, C2h (l = 0) C O 1.017 1.495 1.247 1.310 1.385 1.488 1.448 124.0 126.9 1.098 129.0 131.4 1.085 108.9 167.9 2, D2h (l = 1) C C C C C C C C C C C O O O O O O O 1.194 1.275 1.091 1.487 1.410 124.7 129.1 109.8 172.7 1.247 1.327 1.474 1.450 1.371 118.3 123.9 124.9 147.5 0.994 1.689 Dihedral angle CCCC 61.6° C C C C C C O O O O 3, C2 (l = 0) Figure 1 Geometry parameters of structures 1–3 calculated by the MP2(fc)/6-31G** method.Bond lengths and angles are given in angströms and degrees, respectively. Table 1 Total energies (Etot in hartree), relative energies (DE in kcal mol–1), the number of negative hessian eigenvalues (l), harmonic zero-point correction (ZPE in hartree), relative energy including harmonic zero-point correction (DEZPE in kcal mol–1), reaction enthalpy (DH in kcal mol–1) and the smallest or imaginary vibration frequency (w1/iw in cm–1) for the structures 1–3 calculated by the MP2(fc)/6-31G** method.Structure Etot DE l ZPE DEZPE DH (w1/iw) 1, C2h –531.63149 0 0 0.11877 0 0 22.7 2, D2h –531.62561 3.68 1 0.11193 –0.60 –0.91 i1189.5 3, C2 –531.62732 2.62 0 0.11813 2.21 2.47 46.1Mendeleev Communications Electronic Version, Issue 4, 1998 (pp. 129-168) References 1 C. W. Bock, J. Chem. Phys., 1986, 85, 5391. 2 G. Scherer and H.-H. Limbach, J. Am. Chem. Soc., 1994, 116, 1230. 3 M. K. Holloway, C. H. Reynolds and M. K. Merz, J. Am. Chem. Soc., 1989, 111, 3466. 4 H. Rumpel and H.-H. Limbach, J. Am. Chem. Soc., 1989, 111, 5429. 5 V. Barone and C. Adamo, Chem. Phys. 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