Quantification of thermal ring flexibilities of aromatic and heteroaromatic compounds

The consequences of thermal fluctuations occurring at room temperatures on the aromatic character of a broad group of compounds were analyzed in three distinct ways. First of all, the ring deformations were modeled along normal coordinates coming from quantum thermo-chemistry computations. The amplitudes of vibrations were estimated according to absorbed energies at room temperature. Alternatively, in-plane and out-of-plane ring deformations were modeled via scanning procedure with partial relaxation of the molecular geometry. The influence of ring deformations on π–electron delocalization was expressed in terms of HOMA values. Besides, the ring deformability was defined as the averaged change of bond angles or dihedral angles constituting the ring that was associated with 1.5 kcal mol-1 increase of the system energy. The molecules structures adopted during vibrations at room temperature can lead to significant heterogeneity of structural index of aromaticity. The broad span of HOMA values was obtained for analyzed five- or six-membered aromatic and heteroaromatic rings. However, the averaged values obtained for such fluctuations almost perfectly match HOMA values of molecule in the ground state. It has been demonstrated that the ring deformability imposed by bond angle changes is much smaller than for dihedral angles with the same rise of system energy. Interestingly in the case of out-of-plane vibrations modeled by scanning procedure there is observed linear correlation between ring deformability and HOMA values. Proposed method for inclusion of thermal vibrations in the framework of π–electron delocalization provides natural shift of the way of thinking about aromaticity from a static quantity to a dynamic and heterogeneous one due to inclusion of a more realistic object of analysis – thermally deformed structures. From this perspective the thermal fluctuations are supposed to be non-negligible contributions to aromaticity phenomenon

An ab initio and AIM investigation into the hydration of 2-thioxanthine

<p>Abstract</p> <p>Background</p> <p>Hydration is a universal phenomenon in nature. The interactions between biomolecules and water of hydration play a pivotal role in molecular biology. 2-Thioxanthine (2TX), a thio-modified nucleic acid base, is of significant interest as a DNA inhibitor yet its interactions with hydration water have not been investigated either computationally or experimentally. Here in, we reported an <it>ab initio </it>study of the hydration of 2TX, revealing water can form seven hydrated complexes.</p> <p>Results</p> <p>Hydrogen-bond (H-bond) interactions in 1:1 complexes of 2TX with water are studied at the MP2/6-311G(d, p) and B3LYP/6-311G(d, p) levels. Seven 2TX^...H₂O hydrogen bonded complexes have been theoretically identified and reported for the first time. The proton affinities (PAs) of the O, S, and N atoms and deprotonantion enthalpies (DPEs) of different N-H bonds in 2TX are calculated, factors surrounding why the seven complexes have different hydrogen bond energies are discussed. The theoretical infrared and NMR spectra of hydrated 2TX complexes are reported to probe the characteristics of the proposed H-bonds. An improper blue-shifting H-bond with a shortened C-H bond was found in one case. NBO and AIM analysis were carried out to explain the formation of improper blue-shifting H-bonds, and the H-bonding characteristics are discussed.</p> <p>Conclusion</p> <p>2TX can interact with water by five different H-bonding regimes, N-H^...O, O-H^...N, O-H^...O, O-H^...S and C-H^...O, all of which are medium strength hydrogen bonds. The most stable H-bond complex has a closed structure with two hydrogen bonds (N(7)-H^...O and O-H^...O), whereas the least stable one has an open structure with one H-bond. The interaction energies of the studied complexes are correlated to the PA and DPE involved in H-bond formation. After formation of H-bonds, the calculated IR and NMR spectra of the 2TX-water complexes change greatly, which serves to identify the hydration of 2TX.</p

Imidotitanium Tris(pyrazolyl)hydroborates: Synthesis, Solution Dynamics, and Solid-State Structure.

Reaction of [Ti(NBu(t))Cl(2)(py-Bu(t))(2)] (1; py-Bu(t) = 4-tert-butyl pyridine) with 1 equivalent of K[Tp(Me2)], K[Tp(Pri)] or K[Tp(Pri,Br)] affords the corresponding complexes [Tp(Me2)Ti(NBu(t))Cl(py-Bu(t))] (2), [Tp(Pri)Ti(NBu(t))Cl(py-Bu(t))] (3), and [Tp(Pri,Br)Ti(NBu(t))Cl(py-Bu(t))] (4), respectively, which are the first examples of imido Group 4 tris(pyrazolyl)hydroborates [Tp(Me2) = tris(3,5-dimethylpyrazolyl)hydroborate; Tp(Pri) = tris(3-isopropylpyrazolyl)hydroborate; Tp(Pri,Br) = tris(3-isopropyl-4-bromopyrazolyl)hydroborate]. Complexes 2-4 are fluxional on the (1)H and (13)C NMR time scales, the spectra indicating restricted rotation about the Ti-py-Bu(t) bond. Activation parameters for this dynamic process have been determined both by (13)C NMR lineshape analysis and by coalescence measurements. The solution-state structure for 2 has been unambiguously assigned from a low temperature, phase-sensitive (1)H NOESY DQF spectrum and the solid-state X-ray crystal structure of the dichloromethane solvate of 3 has been determined (space group P2(1)/n; a = 12.539(3), b = 14.686(3), c = 21.747(4) Å; beta = 91.28(3) degrees; R(1) = 0.0694 and wR(2) = 0.154 for 1578 observed reflections). (13)C NMR Deltadelta values (Deltadelta = delta(C(alpha)) - delta(C(beta))) for the tert-butyl imido ligand in 2-4 suggest that the donor ability of the tris(pyrazolyl)hydroborate ligands increases in the order Tp(Pri,Br) &lt; Tp(Pri) &lt; Tp(Me2). None of these ligands, however, is as effective a donor to the metal center as either eta-C(5)H(5) or eta-C(5)Me(5)

Synthesis and imido-group exchange reactions of tert-butylimidotitanium complexes

Treatment of TiCl4 with ButNH2 (6 equivalents) followed by addition of bpy (2 equivalents, 4-/er/-butylpyridine) or pyridine (py) (2 or 4 equivalents) afforded the five- or six-co-ordinate /erf-butylimido complexes [ri(NBut)Cl2(bpy)2] or [Ti(NBut)Cl2(py)n] ( = 2 or 3) respectively in good yields. Reaction of rri(NBu')Cl2(py)3] with RNH2 gave the corresponding arylimido derivatives [Ti(NR)Cl2(py)3] (R = Ph, C6H4Me-4, C6H4NO2-4, C6HjMe2-2,6 or C6H3Pr'2-2,6). Addition of tmen or pmdien to [Ti(NBu')Cl2(py)J (n = 2 or 3 respectively) gave the corresponding bi- or tri-dentate amine complexes, [Ti(NBu')Cl2(tmen)] or [TXNBu'JCl2(pmdien)] (tmen = Me2NCH2CH2NMe2, pmdien = N,N,N′,N″,N″-pentamethyldiethylenetriamine). Complexes labelled have been crystallographically characterised