Singlet State Potential Energy Surfaces Research Articles

Photoisomerization of heptamethine cyanine (Cy7) dyes: A theoretical study

In this study, density functional theory (DFT) combined with time-dependent (TD) DFT calculations were employed to investigate the photoisomerization reaction kinetics of two near infrared (NIR) heptamethine cyanine (Cy7-NH3 and Cy7-SO3) dyes in the ground singlet state and the first excited singlet state. We found that the photoisomerization of the ground state all-trans Cy7 molecules results in at least one mono-cis and one all-cis species that demonstrate redshifted emission, in agreement with recently published transient state excitation modulation spectroscopy and fluorescence correlation spectroscopy measurements. The transition states were estimated for a whole photoisomerization pathway for both the ground singlet and first excited singlet state potential energy surfaces. We have found that all-cis isomers of the studied Cy7 dyes can be achieved through a sequential two-step photoisomerization within the excited singlet state potential energy surface, along the double CC bond adjacent to edge group (leading to mono-cis isomer 1) and along the double CC bond adjacent to the central-chain group (leading to mono-cis isomer 2). Computations show that all-trans→ mono-cis isomer 1→all-cis kinetics is limited by the first trans→ mono-cis isomer 1 stage, while the all-trans→ mono-cis isomer 2→all-cis pathway is limited by the second mono-cis isomer 2→all-cis stage. Accounting for the fact that mono-cis isomer 2 demonstrates red-shifted emission compared to the all-trans form and that this mono-cis isomer 2 is reachable through the energetically favorable all-trans→ mono-cis isomer 2 stage, we concluded that the experimentally observed red-shifted emission by Cy7-NH3 and Cy7-SO3 should be assigned to the formation of mono-cis isomer 2 species. If the all-cis isomer is populated through the double-step photoisomerization it can also be considered as a source of red-shifted emission. However, as follows from our simulations, the all-cis isomer is kinetically intricate to achieve compared to the mono-cis isomer 2.

QM/MM Modeling of the Flavin Functionalization in the RutA Monooxygenase

Oxygenase activity of the flavin-dependent enzyme RutA is commonly associated with the formation of flavin-oxygen adducts in the enzyme active site. We report the results of quantum mechanics/molecular mechanics (QM/MM) modeling of possible reaction pathways initiated by various triplet state complexes of the molecular oxygen with the reduced flavin mononucleotide (FMN) formed in the protein cavities. According to the calculation results, these triplet-state flavin-oxygen complexes can be located at both re-side and si-side of the isoalloxazine ring of flavin. In both cases, the dioxygen moiety is activated by electron transfer from FMN, stimulating the attack of the arising reactive oxygen species at the C4a, N5, C6, and C8 positions in the isoalloxazine ring after the switch to the singlet state potential energy surface. The reaction pathways lead to the C(4a)-peroxide, N(5)-oxide, or C(6)-hydroperoxide covalent adducts or directly to the oxidized flavin, depending on the initial position of the oxygen molecule in the protein cavities.

Mechanism of Synergistic Cu(II)/Cu(I)-Mediated Alkyne Coupling: Dinuclear 1,2-Reductive Elimination after Minimum Energy Crossing Point.

An in-depth theoretical study of synergistic Cu(II)/Cu(I)-mediated alkyne coupling was performed to reveal the detailed mechanism for C-C bond formation, which proceeded via an unusual dinuclear 1,2-reductive elimination. Because the reactant for dinuclear 1,2-reductive elimination was calculated to be triplet while the products were singlet, the minimum energy crossing point (MECP) was introduced to the Cu/TMEDA/alkyne system to clarify the spin crossing between triplet state and singlet state potential energy surfaces. Computational results suggest that C-H bond cleavage solely catalyzed by the Cu(I) cation is the rate-determining step of this reaction and Cu(II)-mediated dinuclear 1,2-reductive elimination after the MECP is a facile process. These conclusions are in good agreement with our previous experimental results.

Journey through the Potential Energy Surfaces for the Isomerization and Decomposition Reactions of the Telluroformaldehyde Analogues: H2A═Te and HFA═Te (A = C, Si, and Ge)

The unavailability of monomeric heavy ketone analogues has been ascribed to the evanescence of the very reactive A═E double bond (A and E are the heavier group 14 and group 16 elements, respectively). Can the isolation of any of the monomeric telluro-ketones be assisted by an energetic favorability on its potential energy surface (PES)? In this light, the reaction pathways for the isomerization and decomposition reactions of H2A═Te and HFA═Te (A = C, Si, and Ge) molecules on their singlet state PES have been studied using second-order Møller-Plesset perturbation theory (MP2). The barrier heights reported suggest that telluroformaldehyde, silanetellone, and germatellone are kinetically more resistant to unimolecular reactions than the corresponding lighter chalcogen analogues. However, upon replacing a hydrogen atom by fluorine, the barrier heights of most of the isomerization and decomposition reactions are lowered. Among the unimolecular reactions studied for the H2A═Te and HFA═Te (A = C, Si, and Ge) molecules, the decomposition of cis-FGeTeH into HF and GeTe is found to be the most facile reaction, with a barrier height of only 4.6 kcal/mol. We also predict the ground state telluro-ketones to be viable molecules, as they have no imaginary vibrational frequencies and their lowest vibrational frequencies are always >100 cm(-1). In view of the scarcity of information on the chemistry of the mentioned telluro-ketones, the molecular parameters of various isomers and decomposition products have been reported, and should be useful for future experimental investigations.

Luminescence of BODIPY and Dipyrrin: An MCSCF Comparison of Excited States

Low lying electronic states of the highly fluorescent BODIPY (boron dipyrromethene, 1) and its nonemissive cousin dipyrrin (2) were investigated by state-of-the-art quantum chemical methods. The opposed luminescence of 1 and 2 is explained by discovering distinct structural and energetic features for the intersection of the ground and first excited singlet state potential energy surfaces, S(0) and S(1). In accessing the intersection region, a B-N σ-bond in 1 has to be broken-an energetically prohibitive change on the nonemissive decay channel. On the contrary, 2 is deactivated via an energetically accessible S(0)/S(1) intersection point. Details of S(0), S(1), S(2), and T(1) wave functions for various regions of the potential energy surfaces were described. Unnoted features for multidimensional vectors that represent S(0) → S(1) and S(0) → T(1) transitions are reported. These correlations regarding S(0) → S(1) and S(0) → T(1) multidimensional vectors were also shown to apply to two highly fluorescent molecules: indole and coumarin.

Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: experiment validates theory.

The O((3)P)+C(2)H(4) reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H+CH(2)CHO, H+CH(3)CO, H(2)+CH(2)CO, CH(3)+HCO, CH(2)+CH(2)O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH(3)+HCO and H+CH(2)CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.

Theoretical study on the gas-phase reaction mechanism between palladium monoxide and methane

The gas-phase reaction mechanism between palladium monoxide and methane has been theoretically investigated on the singlet and triplet state potential energy surfaces (PESs) at the CCSD(T)/AVTZ//B3LYP/6-311+G(2d, 2p), SDD level. The major reaction channel leads to the products PdCH(2) + H(2)O, whereas the minor channel results in the products Pd + CH(3)OH, CH(2)OPd + H(2), and PdOH + CH(3). The minimum energy reaction pathway for the formation of main products (PdCH(2) + H(2)O), involving one spin inversion, prefers to start at the triplet state PES and afterward proceed along the singlet state PES, where both CH(3)PdOH and CH(3)Pd(O)H are the critical intermediates. Furthermore, the rate-determining step is RS-CH(3) PdOH → RS-2-TS1cb → RS-CH(2)Pd(H)OH with the rate constant of k = 1.48 × 10(12) exp(-93,930/RT). For the first C-H bond cleavage, both the activation strain ΔE(≠)(strain) and the stabilizing interaction ΔE(≠)(int) affect the activation energy ΔE(≠), with ΔE(≠)(int) in favor of the direct oxidative insertion. On the other hand, in the PdCH(2) + H(2) O reaction, the main products are Pd + CH(3)OH, and CH(3)PdOH is the energetically preferred intermediate. In the CH(2)OPd + H(2) reaction, the main products are Pd + CH(3)OH with the energetically preferred intermediate H(2)PdOCH(2). In the Pd + CH(3)OH reaction, the main products are CH(2)OPd + H(2), and H(2)PdOCH(2) is the energetically predominant intermediate. The intermediates, PdCH(2), H(2) PdCO, and t-HPdCHO are energetically preferred in the PdC + H(2), PdCO + H(2), and H(2)Pd + CO reactions, respectively. Besides, PdO toward methane activation exhibits higher reaction efficiency than the atom Pd and its first-row congener NiO.

Why do disilanes fail to fluoresce?

In contrast to longer peralkylated oligosilanes, many of which fluoresce efficiently, disilanes and trisilanes exhibit no detectable fluorescence even at low temperatures. This is especially striking in the case of disilanes, whose S1-S0 transition is quite strongly allowed, and which must have very efficient electronic excited state deactivation mechanisms. To identify them, we examine the lowest excited singlet state potential energy surface S1 of Si2Me6 with TDDFT (B3LYP/TZVP, PBE0/TZVP and BHLYP/TZVP) and ab initio (RICC2/TZVP and RIADC(2)/TZVP) methods and identify several shallow minima and nearby funnels. Relaxed excited state structures show strong valence rehybridization relative to the ground state, allowing optimal accomodation of the simultaneous presence of a negative and a positive charge in their Lewis structures. Efficient decay pathways and relations to longer oligosilanes are discussed.

Theoretical study of the activation of CH4–nFn (n=1–3) molecules by platinum in the gas-phase

The gas-phase reactions of fluorocarbon compounds CH4 – nFn (n = 1–3) with Pt (3D, 1S) have been systematically explored via density functional theory (DFT) in order to investigate the mechanisms of these reactions. The results indicate that a reaction of CH3F with Pt (3D, 1S) experiences a rearrangement process to generate counterintuitive production (CH2F−PtH). CH2F2 and CHF3 activation by Pt (3D, 1S) yields high-oxidation-state complexes with carbon–metal double bonds. Moreover, the attack of platinum atoms on fluorine atoms in different fluorocarbon compounds involves intersystem crossing (ISC) between triplet and singlet state Potential Energy Surfaces (PESs). The crossing points (CPs) have been located by the intrinsic reaction coordinate (IRC) approach used by Yoshizawa et al. and corresponding minimum energy crossing points (MECPs) obtained by the mathematical algorithm proposed by Harvey et al. have also been used. Additionally, possible spin inversion processes are discussed using spin–orbit coupling (SOC) calculations.

Theoretical study on the gas‐phase reaction mechanism between nickel monoxide and methane for syngas production

The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH(3) is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH(3)NiOH and HNiOCH(3), locate on the deep well, while CH(3)NiOH is more energetically favorable than HNiOCH(3) on both the triplet and singlet states. The main products shall be syngas once HNiOCH(3) is created on the singlet state, whereas the main products shall be methyl radical if CH(3)NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH(3) formation involving an oxidative addition of NiO molecule into the C-H bond of methane, with an energy barrier of 120.3 kJ mol(-1). The syngas formation would be more effective under higher temperature and photolysis reaction condition.

Improved direct diabatization and coupled potential energy surfaces for the photodissociation of ammonia

Diabatic potential energy surfaces are a convenient starting point for dynamics calculations of photochemical processes, and they can be calculated by the fourfold way direct diabatization scheme. Here we present an improved definition of the reference orbital for applying the fourfold way direct diabatization scheme to ammonia. The improved reference orbital is a geometry-dependent hybrid orbital that allows one to define consistent dominant configuration lists at all geometries important for photodissociation. Using diabatic energies calculated with the new reference orbital and consistent dominant configuration lists, we have refitted the analytical representations of the ground and the first electronically excited singlet-state potential energy surfaces and the diabatic coupling surface. Improved functional forms were used to reproduce the experimental dissociation energies and excitation energies, which will be important for subsequent simulations of photochemical dynamics. We find that the lowest-energy conical intersection point is at 5.16 eV, with C 2v symmetry.

Theoretical studies on the reactions of NO 2 with CH 3SO, CH 3SS, CH 3SO 2 and CH 3S(S)O

The reactions of NO 2 with CH 3SO, CH 3SS, CH 3SO 2 and CH 3S(S)O have been studied at MP2/6-311++G(d, p)//B3LYP/6-311++G (d, p) level. Geometries of the reactants, intermediates (IM), transition states (TS) and products have been optimized and geometries of the transition states are found for the first time. The different structures among four transition states have been discussed by the method of topological analysis of electronic density. The calculated results show that these reactions can proceed via singlet-state or triplet-state potential energy surface (PES). Because of the high energy barrier of triplet PES, the singlet PES reaction is dominant. The reaction mechanisms of NO 2 with CH 3SO, CH 3SS, CH 3SO 2, CH 3S(S)O are similar. On the singlet-state PES, one of O atom of NO 2 attacks the reactants first formed an intermediate, then the intermediate decomposed to CH 3SO 2, CH 3S(S)O, CH 3SO 3, CH 3S(S)O 2 and NO. And because of the low dissociation energy of the processes, these four reactions could occur easily.

Different catalysis role of in‐loop and out‐of‐loop waters in assisting HNS/HSN proton transfer isomerizations: Bridging vs. surrounding effect

AbstractIn this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out‐of‐loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol−1 at the B3LYP/6‐311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol−1 compared with the no‐water‐assisting isomerization barrier T (65.52/43.87 kcal · mol−1). But when adding one water molecule on the HNS, there is another special proton‐transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol−1. Clearly, this process is more difficult to follow than the R–T–P process. For the two‐water‐assisting mechanism, the heat of reaction is 19.61 kcal · mol−1, and the forward/backward barriers are 32.27/12.66 kcal · mol−1, decreased by 33.25/31.21 kcal · mol−1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol−1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol−1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol−1 than the trihydration situation. In the second mode, the outer‐sphere water effect from the other water molecules directly H‐bonded to the loop is considered. When one to three water molecules attach to the looped water in one‐water in‐loop‐assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol−1 compared with the original monohydration‐assisting case. When adding one or two water molecules on the dihydration‐assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in‐loop water‐assisting case are the lowest, and the surrounding water molecules (out‐of‐loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Ab Initio Kinetics for the Unimolecular Reaction C6H5OH → CO + C5H6

The unimolecular decomposition of C(6)H(5)OH on its singlet-state potential energy surface has been studied at the G2M//B3LYP/6-311G(d,p) level of theory. The result shows that the most favorable reaction channel involves the isomerization and decomposition of phenol via 2,4-cyclohexadienone and other low-lying isomers prior to the fragmentation process, producing cyclo-C(5)H(6) + CO as major products, supporting the earlier assumption of the important role of the 2,4-cyclohexadienone intermediate. The rate constant predicted by the microcanonical RRKM theory in the temperature range 800-2000 K at 1 Torr--100 atm of Ar pressure for CO production agrees very well with available experimental data in the temperature range studied. The rate constants for the production of CO and the H atom by O-H dissociation at atmospheric Ar pressure can be represented by k(CO) = 8.62 x 10(15) T(-0.61) exp(-37,300/T) s(-1) and k(H) = 1.01 x 10(71) T(-15.92) exp(-62,800/T) s(-1). The latter process is strongly P-dependent above 1000 K; its high- and low-pressure limits are given.

Water-Assisting Proton Transfer Isomerization of the HNO/HON System in the Singlet State: On the Number of the Effective Water Molecules

A density function theory study is presented for the HNO ⇌ HON isomerization assisted by m water molecules (m = 1−4) on the singlet state potential energy surface. Two modes are considered to model the catalytic effect of m water molecules: (1) water molecule(s) directly participate in forming a proton transfer loop with HNO/HON species and (2) water molecules are out-of-loop, modeling the outer-sphere water effect from the other water molecules directly H-bonded to the loop (referred to as out-of-loop waters). Two mechanisms are proposed for the mono-water-assisting isomerizations and one for each of the other cases. The reactant and product of all groups have been characterized for all potential energy surfaces. For the monohydration mechanism, the reactant complex is connected to the product complex via two determined saddle points, and the reaction heat is 35.5 kcal·mol-1 at the B3PW91/6-311++G** level. The corresponding forward/backward barrier lowerings are obtained as 7.4/1.2 (AT2) and 34.1/28.3 (...

Theoretical study of the ground and first excited singlet state potential energy surfaces of disulphur monoxide (S2O)

An extended region of the ground state potential energy surface (PES) in the vicinity of the experimentally observed isomer of disulphur monoxide (S2O) has been characterized using second-order generalized Van Vleck perturbation theory (GVVPT2) and the cc-pVDZ basis set. The corresponding region of the PES of the first excited singlet state, [Ctilde] 1A′, was analysed in order to elucidate the nature of the experimentally observed dissociation of the excited state S2O. Our calculations support experimental results suggesting the existence of several quasi-bound vibrational energy levels on the excited state PES. The second excited singlet state, [Dtilde] 1A′, was also examined. Our results corroborate the hypothesis that the cause of the predissociation of excited state S2O into its ground state dissociation products is due to the interaction of the lowest two excited state PESs at elongated S-S bond distances. Additional GVVPT2 calculations for the ground and excited state equilibrium structures were performed with the cc-pVTZ basis, and are in good agreement with experimental and CI results.

Theoretical characterization of the disilaethynyl anion (Si2H−)

The singlet-state potential energy surface of the disilaethynyl anion (Si2H−) has been investigated using ab initio self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets. Four stationary points [cyclic (monobridged) A11 (C2v), linear Σ1 + (C∞v), bent A′1 (Cs), and quasilinear Σ1 + (Cs) structures] were located with the correlated wave functions, while only two stationary points [cyclic (monobridged) A11 (C2v) and linear Σ1 + (C∞v) structures] were found with the SCF method. The cyclic structure (C2v) is predicted to be the global minimum at all levels of theory. The linear structure (C∞v) is found to be a transition state between the two quasilinear structures (Cs) at the correlated levels of theory, while the SCF linear structure is predicted to be a transition state between the two cyclic structures. The quasilinear structure possesses a Si–Si–H bond angle similar to that of the monobridged Si2H2 molecule. The bent geometry is assigned to a transition state for the isomerization reaction between the cyclic and quasilinear structures. With the most reliable level of theory, augmented correlation-consistent polarized valence quadruple-ζ CCSD(T), the quasilinear structure is predicted to be 8.6 kcal/mol [7.9 kcal/mol with the zero-point vibrational energy (ZPVE) correction] above the cyclic (monobridged) structure, and the energy barrier for the cyclic→quasilinear isomerization reaction is determined to be 12.1 kcal/mol (11.0 kcal/mol with the ZPVE correction). The inversion reaction between the quasilinear and linear structures is found to have a very small energy barrier. With the estimated aug-cc-pCVQZ CCSD(T) method the electron affinity of Si2H is predicted to be 2.31 eV, which is in excellent agreement with the experimental value 2.31±0.01 eV.

Density functional studies on the mechanisms of unimolecular reactions of HXCSe (X=H, F, Cl, and Br)

Reaction pathways for the decomposition of HXCSe (X=H, F, Cl, and Br) on the singlet state potential energy surface have been studied using the B3LYP/6-311G* level of theory. Predicted molecular parameters (equilibrium geometries, dipole moments, atomic charges, and rotational constants) and vibrational IR spectra agree very well with the available experimental data. Five different reaction mechanisms are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H and XCSe radical formation, and (E) X and HCSe radical formation. From a consideration of the effect of halogen substitution, the following conclusions emerge: our theoretical findings suggest that selenocarbonyl molecules should be both kinetically and thermodynamically stable with respect to the unimolecular decomposition reactions given above. We also report theoretical predictions of molecular parameters and vibrational IR spectra of some monohalogen substituted selenoformaldehydes, which should be useful for further experimental observations.

Cyclobutene Photochemistry. Steric Effects on the Photochemical Ring Opening of Alkylcyclobutenes

Quantum yields for photochemical ring opening and cycloreversion in hydrocarbon solution have been determined for the direct photolysis (214 nm) of six 1,2-dimethylcyclobutene derivatives which contain methyl groups at C3 and C4 in numbers varying from zero to four. As the hydrogens on C& of the parent compound (1,2- dimethylcyclobutene) are replaced with increasing numbers of methyl groups, the total quantum yield for ring opening increases to a maximum of -0.3 and then decreases with further methyl substitution. The quantum yields for ring opening (t q5tod for the cis-isomer is significantly lower, but both yield an approximate 1:l mixture of formally allowed and forbidden diene isomers. A similar trend is observed in the relative quantum yields for ring opening and cycloreversion throughout the series. The results are interpreted in terms of a combination of bond strength and steric effects on the efficiency of the ring-opening process. Increasing methyl substitution causes an increase in @total through the first three members of the series owing to progressive weakening of the C3-C4 bond. Compounds containing cis-dimethyl substitution exhibit substantially reduced quantum yields for ring opening, relative to what would be expected based on bond strength effects alone. This is proposed to be due to steric effects on the efficiency of the process, suggesting that the initial stages of the photochemical ring opening of cyclobutene involve disrotatory motions on the excited singlet state potential energy surface.

The remarkable monobridged structure of Si2H2

Inspired by the observation of a monobridged structure of Si2H2 by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si2H2 singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled-cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple-zeta 2df (TZ2df ) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si2H2 isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure—which exhibits the shortest SiSi bond distance of any molecule characterized to date—and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data. We have located transition states between these pairs of minima—disilavinylidene H2SiSi and monobridged Si(H)SiH; monobridged and dibridged Si(H2)Si; trans-HSiSiH and monobridged. We predict Si(H)SiH to lie 8.7 kcal mol−1 above Si(H2)Si, with the transition state between them 3.7 kcal mol−1 higher. H2SiSi is predicted to lie 11.6 kcal mol−1 above Si(H2)Si and the transition state barrier between H2SiSi and Si(H)SiH is 2.4 kcal mol−1 above H2SiSi. Predictions of absolute 0 K heats of formation for the various structures are presented.