Relative Energy Ordering Research Articles

Three- versus four-coordinate phosphorus in the gas phase and in solution: Treacherous relative energies for phosphine oxide and phosphinous acid

Previous ab initio studies have consistently predicted phosphine oxide (H3PO) to be less stable than its nearly isoenergetic cis- and trans-phosphinous acid isomers (H2POH). However, complete basis set extrapolations employing the coupled-cluster series show that phosphine oxide is actually ca. 1.0 kcal/mol more stable than its acid forms in the gas phase. Incorporation of tight d functions via Dunning’s core-valence (cc-pCVXZ) or newly constructed “plus d” [cc-pV(X+d)Z] basis sets is essential for rapid convergence of core polarization effects which are evident even at the SCF level. The precision to which the phosphorus hybridization is described in the three- and four-coordinate environments ultimately determines the predicted gas-phase relative energy orderings. Focal-point analyses demonstrate that this system represents a disturbing case where use of a conventional valence quadruple-ζ quality basis set (cc-pVQZ)—even at the CCSD(T) level—fails to provide the correct relative energy ordering for simple closed-shell species which do not exhibit appreciable multireference character. Thus, we underscore the importance of using phosphorus basis sets which have the flexibility to describe core polarization adequately. In addition, Monte Carlo (MC) free-energy perturbation simulations in solution clearly demonstrate that the small energy gap significantly increases in favor of the oxide (10.0 kcal/mol) upon solvation due to stronger hydrogen bonding with the highly polar Pδ+→Oδ− bond.

New Ru(II) chromophores with extended excited-state lifetimes.

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.

Concerning the solvent effect in the tautomerism of uracil, 5-fluorouracil, and thymine by density-functional theory and ab initio calculations

The tautomerism of uracil, 5-fluorouracil, and thymine has been investigated in the gas phase and in solution. Electron correlation effects were included in ab initio computations at the MP2 level, and DFT calculations were performed using the B3LYP level. Full geometry optimizations were conducted at the HF/6-31G**, HF/6-31+G**, and B3LYP/6-31+G** levels. Single-point MP2/6-31+G** calculations were performed on the HF/6-31+G** optimized geometries. The influence of the solvent was examined from self-consistent reaction field calculations performed with e=2.21 (1,4-dioxane) and e=78.54 (water). The calculated relative free energies (ΔG) indicate that substitution of uracil at the position group does not change the relative free energy order of the uracil tautomers in the gas phase and in 1,4-dioxane (except at the MP2 level) whereas this ordering changes in water. Attachment of a fluorine atom changes the relative free energy order of uracil tautomers in the gas phase and in solution.

Importance of the proximity of solvation: molecular dynamics and free energy perturbation studies of [16]starand and its model with various alkali metal ions

Strong affinity and high selectivity are good characteristics of a host in host–guest chemistry. Because [16]starand has only one conformation, it should serve as a good probe to study fine structure of solvation. We investigated the complexation behavior of [16]starand and its model with various alkali metal ions using free energy perturbation and molecular dynamics methods. The results from gas phase simulations show that the binding energy difference decreases as the radius of metal ion increases, with only a small disparity between the binding energy difference and binding free energy difference. This small entropy effect in the gas phase may be due to the structural rigidity of [16]starand and its model system. When the complexes lie immersed in water, the order of binding free energies reverses, i.e. the binding energy increases as the radius of alkali metal ion increases. This reversal of order should be from solvent effect. To investigate why complexations with larger metal ions are favored in water, we performed radial distribution function (RDF) analyses from alkali ions to water oxygens. Water coordination numbers for the free solvated ions (NW) and for those in complexes (NC) in aqueous solution were obtained from the RDF data. One might rationally anticipate that the relative free energies would correlate well with the ratio of these coordination numbers (NC/NW), but our findings suggest no direct relationship exists. Interestingly, however, the order of relative binding energies does relate to the solvation ratio within a constant radius very near to the ions. These results demonstrate that the proximity of solvation is an important factor to decide the relative binding free energies.

Interactions of Phenol and Indole with Metal Ions in the Gas Phase: Models For Tyr and Trp Side-Chain Binding

The binding energies of a number of metal cations with phenol and indole in the gas phase were studied experimentally by radiative association kinetics analysis, supplemented by density functional calculations. Radiative association kinetics measurements were carried out in the Fourier transform ion cyclotron resonance mass spectrometer. Reaction in most cases resulted in adduct formation often followed by sequential addition of a second neutral molecule to the ion. The association kinetics were analyzed to yield binding energy values either by using a variational transition state theory-based approach, incorporating quantum-chemical calculations of vibrational frequencies and infrared intensities, or by using the semiquantitative standard hydrocarbon approach. The competitive collision-induced dissociation (CID) technique was used to confirm the order of relative binding energies for several complexes. Calculations of the structures and energies of a number of complexes were performed, by means of the B3LYP-density functional approach, both to complement and compare with the experimental binding energies and also to address the question of π versus heteroatom bonding. With phenol, the two binding sites are close in energy for nontransition metals, but the π-site is relatively more favorable for Fe+ and probably Cr+. Experimental, literature, and calculated values were combined to give best-estimate binding energies for a variety of metal ions. For the nontransition-metal ions characterized, phenol binding is similar to benzene binding, whereas for Cr+ and especially Fe+ phenol binding is stronger than benzene binding. With indole, binding is always enhanced by ∼5−10 kcal mol-1 relative to benzene. Formation of dimer ML2+ complexes followed patterns similar to those of previous benzene results, except that phenol dimer complexes with Mg+ and Al+ were unexpectedly observed, suggesting involvement of an oxygen binding site in these cases.

Excited electronic states of carbon disulphide

The near-ultraviolet spectrum of CS2 is studied thoroughly using high level CISTDQ and EOM-CCSD theory with DZP and TZ2P(f) basis sets. Potential energy curves of the first seven singly excited states with respect to the SCS bond angle have been predicted. The relative energy ordering of the excited states as established at the equilibrium geometries using EOM-CCSD theory is: X˜1Σ+g, < a3B2 < b˜3A2(R) < A1A2 < c˜3B2 < B˜1B2 (V) < d˜3A2 < C˜1A2, which is at odds with previous theoretical work. The bent B˜1B2 state arises from the Renner—Teller splitting of a linear 1Δu state into 1B2 and 1A2 states. In disagreement with previous experimental predictions that the 1A2 state is the lower lying component of this splitting, it is firmly established from the high level results in this research that the C˜1A2 state is the higher lying component of this Renner—Teller pair. Equilibrium geometries are determined and shown to agree qualitatively with experiment, save for the B˜1B2 state. From an analysis of the complex rotational structure of the V band the experimental geometry of the B˜1B2 state was determined to be slightly bent (re = 1.544 Å, θe = 160°), whereas the methods employed in this research predict a significantly bent geometry (re = 1.632 Å, θe = 132.1° at EOM-CCSD/TZ2P(f)). Relative transition energies, Te, are predicted, and EOM-CCSD calculations are shown to agree well with available experimental data, and show the assignment of the V and R bands to be correct while the T band is correctly assigned to the A1A2 state and not the lower lying Renner—Teller component of the 1Δu state. The high level CI methods, although yielding qualitatively correct potential energy curves, do not provide quantitatively accurate transition energies, and it is clear that even an exhaustive treatment of the eight valence π electrons is not entirely satisfactory, while EOM-CCSD is established as an accurate method for the prediction of excited state potential curves.

Excited electronic states of carbon disulphide

The near-ultraviolet spectrum of CS2 is studied thoroughly using high level CISTDQ and EOM-CCSD theory with DZP and TZ2P(f) basis sets. Potential energy curves of the first seven singly excited states with respect to the SCS bond angle have been predicted. The relative energy ordering of the excited states as established at the equilibrium geometries using EOM-CCSD theory is: X˜1Σ+g, < a3B2 < b˜3A2(R) < A1A2 < c˜3B2 < B˜1B2 (V) < d˜3A2 < C˜1A2, which is at odds with previous theoretical work. The bent B˜1B2 state arises from the Renner—Teller splitting of a linear 1Δu state into 1B2 and 1A2 states. In disagreement with previous experimental predictions that the 1A2 state is the lower lying component of this splitting, it is firmly established from the high level results in this research that the C˜1A2 state is the higher lying component of this Renner—Teller pair. Equilibrium geometries are determined and shown to agree qualitatively with experiment, save for the B˜1B2 state. From an analysis of the complex rotational structure of the V band the experimental geometry of the B˜1B2 state was determined to be slightly bent (re = 1.544 Å, θe = 160°), whereas the methods employed in this research predict a significantly bent geometry (re = 1.632 Å, θe = 132.1° at EOM-CCSD/TZ2P(f)). Relative transition energies, Te, are predicted, and EOM-CCSD calculations are shown to agree well with available experimental data, and show the assignment of the V and R bands to be correct while the T band is correctly assigned to the A1A2 state and not the lower lying Renner—Teller component of the 1Δu state. The high level CI methods, although yielding qualitatively correct potential energy curves, do not provide quantitatively accurate transition energies, and it is clear that even an exhaustive treatment of the eight valence π electrons is not entirely satisfactory, while EOM-CCSD is established as an accurate method for the prediction of excited state potential curves.

Ultraviolet photoelectron spectra of 2-haloanilines

The He I and He II spectra of 2-haloanilines were measured and interpreted in terms of a composite-molecule model. The assignments have established the relative ionization energy order of the π MOs (associated with the benzene moiety) and the in-plane nitrogen and halogen lone-pairs. Comparison of the He I and He II cross-section ratios, ab initio calculations using the HF-6-31G** model and comparisons of the linear correlation of the in-plane lone-pairs and Allred electronegativities for the halobenzenes and haloanilines reinforced the preferred assignments.

What is the lowest-energy isomer of the C 60 dimer?

The C 60 dimer is studied using fully relaxed Hartree-Fock, local density, and gradient-corrected density functional calculations both for the neutral and doubly charged case (C 120 2−). In the neutral case, the [2 + 2] cycloadduct is lower in energy than the singly bonded (SB) isomer formed by direct covalent bonding between two C 60 molecules, an isomer for which no minimum in the potential energy surface was found at high levels of theory. With the addition of one electron to each C 60 molecule, however, our best calculations indicate that the relative energy ordering between the [2 + 2] and SB structures reverts. Thus, the SB dimer is predicted to be more stable than the [2 + 2] isomer in mono-anionic fullerides.

Triplet State Interactions between Nucleic Acid Bases in Solution at Room Temperature: Intermolecular Energy and Electron Transfer

This study aims to provide information on the fate of triplet energy in DNA by considering the triplet-mediated reactions which occur between nucleic acid bases in solution, at room temperature. Following sensitization of base triplet states by acetone, the subsequent photophysics and photochemistry are highly dependent on the nature of the nucleotide pair under study. By establishing the direction of triplet energy transfer between pairs of mononucleotides, the relative triplet energy ordering was determined under physiologically relevant conditions, i.e. aqueous solution at room temperature. This order (C > U > G > A > T) is in agreement with that previously reported which was obtained at 77 K in rigid media. The absolute value for TMP triplet energy has been estimated as 310 kJ mol-1, based on triplet energy transfer studies involving acetophenone and 3-methoxyacetophenone. Determination of the triplet energy gaps between all mononucleotides, to within 1 kJ mol-1, has allowed an estimation of the absol...

Level structures in the odd-odd nucleus180Re

Experimental data on energy levels of the odd-odd deformed nucleus180Re obtained from radioactive-decay and heavy-ion reaction studies are analysed to deduce spin-parity and configuration assignments for the six observed rotational bands based on the selection rules for fast beta transitions, criteria for the relative-energy ordering of the triplet and singlet bandheads, two-particle-plus-rotor model calculations including Coriolis mixing, rotational energy systematics involving staggering features, and considerations of gyromagnetic ratios, signature splittings and rotational band alignments.

Possibility of bond stretch isomerism in [Cp(CO) 2M] 2(μ-E) complexes (M  Mn, Re, Cr and W; E  S, Se and Te); a molecular orbital study

Extended Hückel calculations have been carried out on [Cp(CO) 2M] 2(μ-E) complexes (M  Mn, Re, Cr and W; E  S, Se and Te) in linear, bent and triangular geometries. The possibility of double well potential and consequent “bond stretch isomerism” in these complexes is analyzed. All the complexes with a 38 valence electron count (VEC) are calculated to have a minimum corresponding to the triangular geometry. The Mn and Cr complexes with a tellurido bridge have an additional minimum for the bent geometry; only the bent structure is known experimentally. The triangular isomers of these two complexes are marginally higher in energy, but separated by substantial barriers. The other M 2E combinations studied adopt a triangular geometry. The difference in the behaviour between these complexes is explained by the fragment molecular orbital approach and by considering the atomic radii of the M and E species. Single point ab initio calculations using the LANL1MB basis set on [cp(CO) 2M] 2(μ-E) (M  Mn and Re; E  S, Se and Te) confirm the relative energy orderings produced by the extended Hückel calculations. The relation between the ME bond orders and bond lengths and the extent of Main Group d-orbital participation is also analyzed. It is predicted that complexes with a VEC of 36 and 40 should adopt linear and bent geometries, respectively.

Photoinduced energy and electron transfer in inorganic covalently linked systems

Photoinduced electron transfer and charge recombination can be studied in metal-polypyridine polynuclear complexes based on the (MebpyCH 2CH 2Mebpy) (Me, methyl; bpy,2,2′-bipyridine) bridging ligand. An example is provided by the binuclear complex [Ru(Me 2phen) 2(MebpyCH 2CH 2Mebpy)Rh(Me 2bpy) 2] 5+ (phen, 1,10-phenanthroline) (abbreviated as Ru(II)Rh(III)). In this system, photoinduced electron transfer processes originating from both local excited states (*Ru(II)Rh(III)→Ru(III)Rh(II) and Ru(II)*Rh(III)→Ru(III)Rh(II)), as well as charge recombination (Ru(III)Rh(II)→Ru(II)Rh(III)), have been resolved in the nanosecond and picosecond time domains using transient absorption and emission measurements. An energy transfer process (Ru(II)*Rh(III)→*Ru(II)Rh(III) can also be observed in rigid media, where the competing electron transfer process is suppressed. The factors affecting the kinetics of the various electron transfer steps can be discussed in terms of current transfer theories. Polychromophoric complexes suited for the study of intercomponent energy transfer can be designed using Re(I) and Ru(II) polypyridine units as molecular components and cyanide bridges as connectors. The energy flow in such systems is determined by the relative energy ordering of the metal-to-ligand charge transfer (MLCT) excited states of the various units, which can be controlled synthetically through the type of metal, the type of polypyridine ligand and the binding mode of the bridging cyanide(s). For example, in [NCRu(bpy) 2CNRu(bpy) 2CN] +, the flow is from the C-bonded to the N-bonded (to bridging cyanide) unit; in [(CO) 3Re(phen)NCRu(bpy) 2CN] +, the flow is from Re(I) to Ru(II); in [NCRu(bpy) 2 CNRu(bpy(COO) 2) 2NCRu(bpy) 2CN] 2−, the flow is from the bpy-containing units to the units containing dicarboxy—bpy. With regard to the identification of the lowest energy state in such systems, valuable information can be obtained using vibrational spectroscopies, such as time-resolved IR and resonance Raman. Compounds of this series behave in many respects as supramolecular antenna systems. Applications of the antenna effect to the spectral sensitization of semiconductors are discussed.

Infrared fingerprints of nine fullerene C82 isomers: a semiempirical prediction

We use a combination of the quantum chemical force field for [pi] electrons (QCFF/PI) method and the modified neglect of differential overlap-parametrization method 3 (MNDO-PM3) to predict some properties of the nine isomers of C[sub 82] that satisfy the isolated pentagon rule. All the structural parameters are optimized with both methods. The two calculations furnish a coherent picture for the relative stabilities, energy ordering, and energy differences among the nine clusters. A C[sub 2] isomer is the most stable while the two C[sub 30] isomers are the least stable. The two methods also agree in the energy trends of the HOMOs and LUMOs. The nine C[sub 82] molecules have HOMOs at higher energies and LUMOs at lower energies than C[sub 60] and C[sub 70]. On a qualitative basis, they are therefore better electron donors and better electron acceptors. The HOMO-LUMO energy gap is smaller for the less stable species, with the energy of the LUMO more strongly affected upon changing the isomer. In an effort to contribute to the identification of the different isomers, we predict the infrared spectra of all nine molecules by a combination of the QCFF/PI force fields and MNDO-PM3 derivatives of the dipole moments. 27 refs.,more » 10 figs., 1 tab.« less

An intermediate neglect of differential overlap model for second‐row transition metal species

AbstractAn intermediate neglect of differential overlap (INDO) model for elements of the second transition series is described. Values of exponents for the radial portion of the Slater basis atomic orbitals, the orbital ionization energies, and the beta (or bonding) parameters for these elements are reported. Use of the model to investigate equilibrium geometries, relative energy ordering of geometric isomers, spectroscopic transition energies, energy ordering of state wave functions of different symmetry, trends in dipole moments, and equilibrium stretching frequencies is reported. The results indicate that the INDO method is capable of yielding quality results for species containing atoms of the second transition series.

ChemInform Abstract: Violation of Hund′s Multiplicity Rule in the Lowest Excited Singlet‐Triplet Pairs of Cyclic Bicalicene and Its Higher Homologues.

The relative energy ordering of the lowest excited singlet–triplet pair in the title molecules has been examined by using the Pariser–Parr–Pople–type SCF MO method and by taking into account configuration interactions. In cyclic bicalicene, the excited singlet state is predicted to lie below the triplet state by about 4 kcal mol–1, at least within this MO approach. This violation of Hund's multiplicity rule is ascribed essentially to the correlation effects brought about by spin polarization terms. Such a violation of Hund's rule is expected to occur in cyclic bisesquifulvalene and cyclic bitriaheptafulvalene of D2h symmetry.

Violation of Hund's multiplicity rule in the lowest excited singlet–triplet pairs of cyclic bicalicene and its higher homologues

The relative energy ordering of the lowest excited singlet–triplet pair in the title molecules has been examined by using the Pariser–Parr–Pople–type SCF MO method and by taking into account configuration interactions. In cyclic bicalicene, the excited singlet state is predicted to lie below the triplet state by about 4 kcal mol–1, at least within this MO approach. This violation of Hund's multiplicity rule is ascribed essentially to the correlation effects brought about by spin polarization terms. Such a violation of Hund's rule is expected to occur in cyclic bisesquifulvalene and cyclic bitriaheptafulvalene of D2h symmetry.

Positive-ion chemistry of cyclopentadienylcobalt dicarbonyl

Ion cyclotron resonance (ICR) techniques are used to study the positive-ion chemistry of CpCo(CO) 2, Cp = η 5-C 5H 5. An order of relative ligand binding energies to CpCo + is established: benzene > MeCN ⋍ NH 3 > tetrahydrofuran ⋍ Me 2CO > propylene ⋍ Me-CHO ⋍ Me 2O ⋍ NO ⩾ MeOH > CO. Relative ligand binding energies to CpCo(CO) + are similar. The proton affinity of CpCo(CO) 2 is indicated to be in the range 208 ∓ 4 kcal mol −1, giving a homolytic bond energy of D(CpCo(CO) 2 +H) = 80 ∓ 6 kcal mol −1. This suggests that protonation occurs on the ring. Methyl bromide reacts with CpCo + to substitute Me- for H- on the cyclopentadienyl ring. This reaction occurs four times and is followed by oxidative bromine transfer to give (η 5-Me 4C 5H)CoBr + as the final product in this sequence. The results observed with CpCo(CO) 2 are generally very similar to those previously reported for CpNi(NO).

Intramolecular hydrogen bonding in ortho-substituted N,N-dimethylbenzamides

Intramolecular hydrogen bonding in o-hydroxy-, o-amino- and o-mercapto- N,N-dimethylbenzamide has been examined with the help of 1H, 13C n.m.r. and infrared spectroscopy, a study of barriers to rotation about the C- N bond, and a study of substituent effects using a multi-substituent parameter equation. The cyclic intramolecular hydrogen-bonded structure for the o-mercapto compound is non-planar, and conformational isomers have been detected. The order of relative energies of the hydrogen bonds formed in o-substituted N,N-dimethylbenzamides is OH &gt; NH2 &gt; SH, as determined from a rotational barrier probe. It has beenverified that intramolecular hydrogen bonding causes a downfield shift of the 13C n.m.r. chemical shift of the carbonyl group in these compounds.

The Zeeman effect in spin-allowed and forbidden doublet-doublet transitions in orthorhombic rotating molecules

Theoretical predictions of the Zeeman effect on spin-allowed and forbidden doublet-doublet transitions in near-symmetric top molecules are reported, with the assistance of computer simulations of band profiles. Bandwidths in the high-field limit have been determined. Marked differences have been found between the behavior of spin-allowed and forbidden transitions. The intermediate-field effect on spin-allowed transitions is sensitive to the relative energy order of the zero-field spin components of the rotational sublevels in the combining vibronic states. In the high-field limit the rotational components of spin-allowed transitions give rise to a single band, corresponding to Δ M S = 0, whereas the bands with Δ M S = ±1 are also expected in spin-forbidden transitions. The applicability to actual cases is finally discussed.