Matus Myrna H, Nguyen Minh Tho, Dixon David A
Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, AL 35487-0336, USA.
J Phys Chem A. 2007 Mar 8;111(9):1726-36. doi: 10.1021/jp067892v. Epub 2007 Feb 14.
The heats of formation of diphosphene (cis- and trans-P2H2), phopshinophosphinidene (singlet and triplet H2PP) and diphosphine (P2H4), as well as those of the P2H and P2H3 radicals resulting from PH bond cleavages, have been calculated by using high-level ab initio electronic structure theory. Energies were calculated using coupled-cluster theory with a perturbative treatment for triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets with additional tight d-functions on P (aug-cc-pV(n+d)Z) up to quadruple- or quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries and vibrational frequencies were determined with the CCSD(T) method. Core-valence and scalar relativistic corrections were included, as well as scaled zero-point energies. We find the following heats of formation (kcal/mol) at 298 [0] K: DeltaH(degree)(f)(P2H) = 53.4 [54.4]; DeltaH(degree)(f)(cis-P2H2) = 32.0 [33.9]; DeltaH(degree)(f)(trans-P2H2) = 28.7 [30.6]; DeltaH(degree)(f)(H2PP) = 53.7 [55.6]; DeltaH(degree)(f)(3H2PP) = 56.5 [58.3]; DeltaH(degree)(f)(P2H3) = 32.3 [34.8]; DeltaH(degree)(f)(P2H4) = 5.7 [9.1] (expt, 5.0 +/- 1.0 at 298 K); and DeltaH(degree)(f)(CH3PH2) = -5.0 [-1.4]. We estimate these values to have an accuracy of +/-1.0 kcal/mol. In contrast to earlier results, we found a singlet ground state for phosphinophosphinidene (H2PP) with a singlet-triplet energy gap of 2.8 kcal/mol. We calculated the heats of formation of the methylated derivatives CH3PPH, CH3HPPH2, CH3PPCH3, CH3HPP, (CH3)2PP, (CH3)2PPH2, and CH3HPPHCH3 by using isodesmic reactions at the MP2/CBS level. The calculated results for the hydrogenation reactions RPPR + H2 --> RHPPHR and R2PP + H2 --> R2PPH2 show that substitution of an organic substituent for H improves the energetics, suggesting that secondary diphosphines and diphosphenes are potential candidates for use in a chemical hydrogen storage system. A comparison with the nitrogen analogues is given. The mechanism for H2-generation from diphosphine without and with BH3 as a catalyst was examined. Including tunneling corrections, the rate constant for the catalyzed reaction is 4.5 x 1015 times faster than the uncatalyzed result starting from separated catalyst and PH2PH2.
利用高水平从头算电子结构理论计算了双膦烯（顺式和反式 - P₂H₂）、膦基亚膦烯（单重态和三重态H₂PP）和双膦（P₂H₄）的生成热，以及由PH键断裂产生的P₂H和P₂H₃自由基的生成热。能量计算采用耦合簇理论并对三重激发进行微扰处理（CCSD(T)），并使用在P上带有额外紧密d函数的增强相关一致基组（aug - cc - pV(n + d)Z）直至四重或五重ζ，以对能量进行完整基组外推。几何结构和振动频率用CCSD(T)方法确定。包括了芯价和标量相对论校正以及缩放的零点能。我们在298 [0] K时得到以下生成热（kcal/mol）：ΔH°(f)(P₂H) = 53.4 [54.4]；ΔH°(f)(顺式 - P₂H₂) = 32.0 [33.9]；ΔH°(f)(反式 - P₂H₂) = 28.7 [30.6]；ΔH°(f)(H₂PP) = 53.7 [55.6]；ΔH°(f)(³H₂PP) = 56.5 [58.3]；ΔH°(f)(P₂H₃) = 32.3 [34.8]；ΔH°(f)(P₂H₄) = 5.7 [9.1]（实验值，298 K时为5.0 ± 1.0）；以及ΔH°(f)(CH₃PH₂) = -5.0 [-1.4]。我们估计这些值的准确度为±1.0 kcal/mol。与早期结果相反，我们发现膦基亚膦烯（H₂PP）的基态为单重态，单重态 - 三重态能隙为2.8 kcal/mol。我们通过在MP2/CBS水平上使用等键反应计算了甲基化衍生物CH₃PPH、CH₃HPPH₂、CH₃PPCH₃、CH₃HPP、(CH₃)₂PP、(CH₃)₂PPH₂和CH₃HPPHCH₃的生成热。氢化反应RPPR + H₂ → RHPPHR和R₂PP + H₂ → R₂PPH₂的计算结果表明，用有机取代基取代H可改善能量学性质，这表明仲双膦和双膦烯是化学储氢系统中潜在的候选物。给出了与氮类似物的比较。研究了在没有催化剂和有BH₃作为催化剂时双膦生成H₂的机理。包括隧道效应校正后，催化反应的速率常数比从分离的催化剂和PH₂PH₂开始的未催化反应结果快4.5×10¹⁵倍。
