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The adsorbed structures, Surface-enhanced Raman scattering (SERS) and pre-resonance Raman spectra of the U-Ag and U〈 〉U-Ag complexes are calculated using density-functional theory (DFT) method with B3LYP/6-311g (d, p) (C, H, O)/Lanl2dz (Ag) basis set. The uracil and uracil dimer prefer to a perpendicular orientation adsorbed on the Ag surface via atom O8 site. The SERS of the U-Ag and U〈 〉U-Ag complexes are similar with the normal Raman spectra of the isolated uracil and uracil dimer but with an enhancement factor less than 10. The enhancement mechanism is ascribed to the ground state static chemical enhancement from the static polarizability changes. The pre-resonance Raman spectra of the complexes are obtained at 316 nm and 315 nm, in which the enhancement factors are up to 103–104. The greatest enhancement factor is observed for the C2=O8 stretching vibration, caused by the electron transfer from the molecules to Ag atom. The charge transfer resonant enhancement plays the major role in the molecule–metal system. [ABSTRACT FROM AUTHOR]

The trinuclear derivative Fe3(C8H8)3 was synthesized in 2009 by Lavallo and Grubbs via the reaction of Fe(C8H8)2 with a bulky heterocyclic carbene. This fascinating structure is the first example of a derivative of the well-known Fe3(CO)12 in which all 12 carbonyl groups have been replaced by hydrocarbon ligands. The density functional theory predicts a structure having a central Fe3 equilateral triangle with 2.9 Å Fe-Fe single bonded edges bridged by η5,η3-C8H8 ligands. This structure is close to the experimental structure, determined by X-ray crystallography. The related hypoelectronic MNi3(C8H8)3 derivatives (M = Cr, V, Ti) are predicted to have central scalene M3 triangles with edge lengths and Wiberg bond indices (WBIs) corresponding to one formal single M-M bond, one formal double M-M bond, and one formal triple M≡M bond. For Mn3(C8H8)3, both a doublet structure with one Mn-Mn double bond and two Mn-Mn single bonds in the Mn3 triangle, and a quartet structure with two Mn-Mn double bonds and one Mn-Mn single bond are predicted. The hyperelectronic derivatives MNi3(C8H8)3 have weaker direct M-M interactions in their M3 triangles, as indicated by both the M-M distances and the WBIs. Thus, NiNi3(C8H8)3 has bis(trihapto) η3,η3-C8H8 ligands bridging the edges of a central approximately equilateral Ni3 triangle with long Ni…Ni distances of ~3.7 Å. The WBIs indicate very little direct Ni-Ni bonding in this Ni3 triangle and thus a local nickel environment in the singlet Ni3(C8H8)3 similar to that observed for diallylnickel (η3-C3H5)2Ni. [ABSTRACT FROM AUTHOR]

Azulene is reported to react with Mn2(CO)10to give trans-C10H8Mn2(CO)6, which has been shown by X-ray crystallography to have a bis(pentahapto) structure with no metal-metal bond. This structure is found by density functional theory to be the lowest energy C10H8Mn2(CO)6structure. However, a corresponding bis(pentahapto) cis-C10H8Mn2(CO)6structure, also without an MnMn bond, lies within ∼1 kcal mol−1of this global minimum. The lowest energy C10H8Mn2(CO)5structure is singlet cis-η5,η5-C10H8Mn2(CO)5with an Mn→Mn dative bond from the Mn(CO)3group to the Mn(CO)2group. However, a singlet cis-η6,η4-C10H8Mn2(CO)5structure with a normal Mn–Mn single bond lies within ∼6 kcal mol−1of this global minimum. The lowest energy structures of the more highly unsaturated C10H8Mn2(CO)n(n= 4, 3, 2) systems all have cisgeometries and manganese-manganese bonds of various orders. The corresponding global minima are triplet cis-η5,η3-C10H8Mn2(CO)3(η2-μ-CO) for the tetracarbonyl with a four-electron donor bridging carbonyl group, singlet cis-η5,η5-C10H8Mn2(CO)3for the tricarbonyl, and triplet cis-η6,η4-C10H8Mn2(CO)(η2-μ-CO) for the dicarbonyl. [ABSTRACT FROM AUTHOR]

The mixed sandwich compounds (η5-C5H5)M(η7-C7H7) (M = Ti, V, Cr) have been known for approximately 50 years. The vanadium derivative ('trovacene') has recently been shown to be a useful precursor for the preparation of a variety of paramagnetic sandwich compounds. In this connection the structures of the complete series of the first row transition metal mixed sandwich compounds C5H5MC7H7 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. The experimentally known parallel ring sandwich structures (η5-C5H5)M(η7-C7H7) are found for the Ti, V, and Cr derivatives. Analysis of the frontier molecular orbitals of these complexes indicates that, for the metal d-orbitals, the z2 orbital is essentially non-bonding, the x2-y2 and xy orbitals are δ bonding to the seven-membered ring, and the xz and yz orbitals are π bonding to the five-membered ring. For the later transition metals, (η5-C5H5)M(η5-C7H7) structures with pentahapto seven-membered rings are preferred for iron and manganese and (η5-C5H5)M(η3-C7H7) structures with trihapto seven-membered rings are preferred for cobalt and nickel, leading to either 17- or 18-electron complexes in all cases. A higher energy parallel sandwich structure (η5-C5H5)Co(η7-C7H7) with a quartet spin state and a formal 21-electron cobalt configuration is also found for the cobalt derivative. [ABSTRACT FROM AUTHOR]

The structures and the relative energies of six possible tautomers of the thymine base have been studied by density functional theory (DFT) using the B3LYP and BP86 functionals. The keto-thymine (T1) is predicted to be the most stable thymine tautomer, which is consistent with the other theoretical results and experimental data. The corresponding thymine cations and anions are studied using the same level of theory with double-ζ plus polarization and diffuse functions (DZP++) basis sets. The ionization potentials (IPs), the electron affinities (EAs), and proton affinities (PAs) for different protonation sites in thymine base are obtained. T1 has the largest ionization potential and the lowest proton affinity among all the considered thymine tautomers. Faisant appel à la théorie de la fonctionnelle de la densité et utilisant des fonctions B3LP et BP86, on a étudié les structures et les énergies relatives de six tautomères potentiels de la thymine à l’été de base. Sur la base de ces calculs, il est prédit que la cétothymine (T1) est le tautomère le plus stable de la thymine; cette conclusion est en accord avec les autres calculs théoriques et avec les résultats expérimentaux. Les cations et anions correspondants à la thymine ont aussi été étudiés aux même niveaux de la théorie en utilisant comme ensembles de base des fonctions de polarisation double ζ plus et des fonctions diffuses (DZP++). On en a extrait les potentiels d’ionisation (PI), les affinités électroniques (AE) et les affinités protoniques (AP) pour les différents sites de protonation de la thymine. La cétothymine (T1) possède le potentiel d’ionisation le plus élevé et l’affinité protonique la plus faible parmi tous les tautomères considérés de la thymine. [ABSTRACT FROM AUTHOR]

The structures, energies, and electron affinities of the tautomers of dehydrogenated thymine radicals (T-H) have been studied theoretically. Geometry optimizations were carried out utilizing the density functional theory (DFT) methods with double-ζ quality basis sets plus polarization and diffuse functions (DZP++). These methods have been carefully calibrated for the prediction of electron affinities. Significant structural differences were found among thymine 24 dehydrogenated radicals and their corresponding anions from thymine tautomers. The electron affinities for these radicals are in the range of 0.96 and 3.88 eV, contrasting with the much smaller electron affinities associated with the closed-shell thymine tautomers. Among these investigated radicals those with removal of a hydrogen atom from the nitrogen atoms present the larger electron affinities (EAs). [ABSTRACT FROM AUTHOR]

Binuclear cycloheptatrienylchromium carbonyls of the type (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η7-C7H7 rings are not found for (C7H7)2Cr2(CO)n structures having two or more carbonyl groups. This result stands in contrast to the related (CnHn)2M2(CO)n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C7H7)2Cr2(CO)n structures have bent trihapto or pentahapto C7H7 rings and Cr≡Cr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C7H7 ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C7H7–Cr bond are predicted for (C7H7)2Cr2(CO)n (n = 6, 4, 2). The global minima found for the (C7H7)2Cr2(CO)n derivatives for n = 6, 5, and 4 are (η5-C7H7)(OC)2Cr≡Cr(CO)4(η1-C7H7), (η3-C7H7)(OC)2Cr≡Cr(CO)3(η2,1- C7H7), and (η5-C7H7)2Cr2(CO)4, respectively. The global minima for (C7H7)2Cr2(CO)n (n = 3, 2) have rearranged C7H7 groups. Singlet and triplet structures with heptahapto η7-C7H7 rings are found for the dimetallocenes (η7-C7H7)2Cr2(CO) and (η7-C7H7)2Cr2, with the singlet structures being of much lower energies in both cases. [Copyright &y& Elsevier]

The binuclear benzenemanganese carbonyls (η 6-C 6H 6) 2Mn 2(CO) n( n= 4, 3, 2, 1) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The singlet doubly bridged and unbridged isomers of the saturated (η 6-C 6H 6) 2Mn 2(CO) 4are nearly degenerate, suggesting a highly fluxional system. The global minimum of (η 6-C 6H 6) 2Mn 2(CO) 3is a triplet electronic statea triply bridged isomer analogous to the known isoelectronic triplet Cp 2Fe 2(μ-CO) 3. The doubly bridged singlet structure (η 6-C 6H 6) 2Mn 2(CO)(μ-CO) 2lies 9 kcal/mol (B3LYP) or 7 kcal/mol (BP86) above this global minimum. The global minimum of (η 6-C 6H 6) 2Mn 2(CO) 2is a doubly bridged structure with a short manganese−manganese distance suggestive of a MnMn triple bond. Singlet and triplet structures are found for (η 6-C 6H 6) 2Mn 2(CO) with a two-electron donor CO group and metal−metal distances suggesting a M−M quadruple bond for the singlet state and a MM triple bond for the triplet state. [ABSTRACT FROM AUTHOR]

In view of experimental work on (η 4-C 4H 4) 2Fe 2(μ-CO) 3the full range of binuclear cyclobutadieneiron carbonyls (η 4-C 4H 4) 2Fe 2(CO) n( n= 5, 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The pentacarbonyl (η 4-C 4H 4) 2Fe 2(CO) 5is predicted to have a singly bridged (η 4-C 4H 4) 2Fe 2(CO) 4(μ-CO) structure with an Fe−Fe single-bond distance of 2.743 Å (BP86). For (η 4-C 4H 4) 2Fe 2(CO) 4cisand transdoubly CO-bridged singlet structures are found as well as two types of doubly CO-bridged triplet structures. One type has Fe−Fe bond distances in the single-bond range 2.64−2.66 Å and 17-electron iron configurations. The other type of triplet structure has FeFe bond distances in the range 2.38−2.39 Å for a σ + 2/ 2π double bond like that in dioxygen and 18-electron iron configurations. For (η 4-C 4H 4) 2Fe 2(CO) 3the global minimum is the experimentally observed triply CO-bridged structure with an FeFe distance of 2.148 Å (BP86), corresponding to the triple bond required for 18-electron iron configurations. For (η 4-C 4H 4) 2Fe 2(CO) 2the doubly CO-bridged triplet structure with an FeFe distance of 2.216 Å is the lowest energy structure. For (η 4-C 4H 4) 2Fe 2(CO) the lowest lying singlet, triplet, and quintet structures have four-electron-donor bridging η 2-μ-CO groups, as indicated by Fe−O distances around 2.0−2.1 Å. Higher energy perpendicular or near-perpendicular structures with bridging C 4H 4ligands are found in addition to coaxial structures for both (η 4-C 4H 4) 2Fe 2(CO) 2and (η 4-C 4H 4) 2Fe 2(CO). [ABSTRACT FROM AUTHOR]

Density functional methods indicate that the global minimum for Cr2(NO)2(CO)8 is a staggered D4d structure in accord with experiment and analogous to the isoelectronic Mn2(CO)10. For the unsaturated Cr2(NO)2(CO)n derivatives the lowest energy structures are very different from the lowest energy structures for the isoelectronic Mn2(CO)n+2 derivatives. Thus the global minimum for Cr2(NO)2(CO)7 is an unbridged structure with a Cr(NO)(CO)4 fragment linked to a Cr(NO)(CO)3 fragment through a CrCr double bond. For Cr2(NO)2(CO)6 the global minimum is a structure with two bridging CO groups, whereas the global minimum for Mn2(CO)8 is an unbridged structure. For Cr2(NO)2(CO)5 both NO groups are bridging NO groups with one of them having a short enough Cr-O distance to be considered a formal five-electron donor η²-μ-NO group. Thus the isoelectronic substitution of NO for CO with a necessary adjustment in the central metal atom can lead to significant shifts in the relative energies of various structural types of metal carbonyl nitrosyls, particularly for unsaturated molecules. For the mononuclear Cr(NO)2-(CO)3 the theoretical structure differs from that deduced from matrix isolation experiments. Moreover, the ν(CO) and ν(NO) vibrational frequencies predicted here for Cr(NO)2(CO)3 correspond more closely with the unassigned species labeled "Cr(NO)(CO)x" in the experiments rather than the species claimed to be Cr(NO)2(CO)3. [ABSTRACT FROM AUTHOR]

The manganese carbonyl nitrosyls Mn(NO)(CO)4, Mn2(NO)2(CO)n (n = 7, 6, 5, 4), and Mn3(NO)3(CO)9 have been studied by density functional theory (DFT) using the B3LYP and BP86 methods for comparison of their predicted structures with those of isoelectronic iron carbonyl derivatives. DFT predicts a trigonal bipyramidal structure for Mn(NO)(CO)4 with an equatorial NO group very close to the experimental structure. The predicted lowest energy structure for Mn2(NO)2(CO)7 has two bridging NO groups in contrast to the known structure of the isoelectronic Fe2(CO)9, which has three bridging CO groups. The structures for the unsaturated binuclear Mn2(NO)2(CO)n (n = 6, 5, 4) derivatives are similar to those of the corresponding binuclear iron carbonyls Fe2(CO)n+2 derivatives but always with a preference of bridging NO groups over bridging CO groups. The trinuclear Mn3(NO)3(CO)9 is predicted to have a structure analogous to the known structure for Fe3(CO)12 but with two bridging NO groups rather than two bridging CO groups across one of the metal-metal edges of the M3 triangle. The dark red solid photolysis product of Mn(NO)(CO)4 characterized by its ν(CO) and ν(NO) frequencies ~45 years ago is suggested by these DFT studies not to be the originally assumed Mn2(NO)2(CO)7 analogous to Fe2(CO)9. Instead, this photolysis product appears to be Mn2(NO)2(CO)5 with a MnMn formal triple bond analogous to (η-C5H5)2V2(CO)5 obtained from the photolysis of (η5-C5H5)V(CO)4. [ABSTRACT FROM AUTHOR]

The article analyzes the characteristic of unsaturation in trinuclear metal carbonyl clusters. The author uses density functional theory (DFT) methods to examine the relative energies of three types of structures for Fe3(CO)12. The unsaturated species Fe3(CO)n (n = 12, 11, 10, 9) are similarly examined in order to determine whether their most favorable isomers use metal-metal multiple bonding.

The binuclear cyclopentadienyliron carbonyls Cp2Fe2(CO)n (n = 4, 3, 2, 1; Cp = η5·C5H5) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The trans- and cis-Cp2Fe2(CO)2(μ-CO)2 isomers of Cp2Fe2(CO)4 known experimentally are predicted by DFT methods to be genuine minima with no significant imaginary vibrational frequencies. The energies of these two Cp2Fe2(CO)2(μ-CO)2 structures are very similar, consistent with the experimental observation of an equilibrium between these isomers in solution. An intermediate between the interconversion of the trans- and cis-Cp2Fe2(CO)2(μ-CO)2 dibridged isomers of Cp2Fe2(CO)4 can be the trans unbridged isomer of Cp2Fe2(CO)4 calculated to be 2.3 kcal/mol (B3LYP) or 9.1 kcal/mol (BP86) above the global minimum trans-Cp2Fe2(CO)2(μ-CO)2. For the unsaturated Cp2Fe2(CO)3, the known triplet isomer Cp2Fe2(μ-CO)3 with an Fe=Fe double bond similar to the OO double bond in O2 is found to be the global minimum. The lowest-energy structure for the even more unsaturated Cp2Fe2(CO)2 is a dibridged structure Cp2Fe2(μ-CO)2, with a short Fe-Fe distance suggestive of the FeFe triple bond required to give both Fe atoms the favored 18-electron configuration. Singlet and triplet unbridged structures for Cp2Fe2(CO)2 were also found but at energies considerably higher (20–50 kcal/mol) than that of the global minimum Cp2Fe2(μ-CO)2. The lowest-energy structure for Cp2Fe2(CO) is the triplet unsymmetrically bridged structure Cp2Fe2(μ-CO), with a short Fe-Fe distance (∼2.1 Å) suggestive of the σ + 2π + ²/2δ Fe(Multiple line equation(s) cannot be represented in ASCII text)Fe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration. [ABSTRACT FROM AUTHOR]