Density functional theory (DFT) has been used to investigate the conformations and thermochemistry on the singlet and triplet potential energy surfaces (PES) of Cr2(CO)10. The global minimum energy structure for the lowest singlet state of C2h symmetry is consistent with a model of two interacting Cr(CO)5 fragments in which one carbonyl in each fragment acts as an asymmetric four-electron donor bridging carbonyl, with chromium-chromium distances of 2.93 A (B3LYP) or 2.83 A (BP86). Avoiding a Cr...Cr bond by incorporating four-electron donor CO groups in this way allows each chromium atom in singlet Cr(CO)10 to attain the favored 18-electron configuration by using, in a simple picture of the bonding, only the six octahedral sp3d2 hybrids. The dissociation energy to two Cr(CO)5 fragments or to Cr(CO)6 + Cr(CO)4 fragments is predicted to be 10 kcal mol(-1). The lowest triplet state of Cr2(CO)10 is predicted to lie approximately 10 kcal mol(-1) above the singlet global minimum. In the case of triplet Cr2(CO)10 the lowest energy minima were found to be of C2 and C2h symmetry, with similar energies. The chromium-chromium distances in triplet Cr2(CO)10 were found to be shorter than those in the corresponding singlet structures, namely 2.81 (B3LYP) or 2.68 A (BP86) suggesting a sigma + 2(1/2) pi Cr=Cr double bond similar to the O=O bond in O2 or the Fe=Fe bond in the experimentally observed triplet state (Me5C5)2Fe2(mu-CO)3.

Density functional theory (DFT) is used to obtain the first structural characterization of the unsaturated dichromium carbonyl Cr2(CO)9, which is predicted to have a remarkably short metal−metal bond length of 2.31 Å (B3LYP) or 2.28 Å (BP86). This chromium−chromium distance is essentially identical to that reported experimentally for the established Cr⋮Cr triple bond in (η5-Me5C5)2Cr2(CO)4. The dissociation energy to the fragments Cr(CO)4 and Cr(CO)5 is determined to be 32 kcal/mol (B3LYP) or 43 kcal/mol (BP86). For comparison, the Cr2(CO)10 molecule and the saturated Cr2(CO)11 system have negligible dissociation energies. The minimum energy Cr2(CO)9 structure is of C s symmetry with the two chromium atoms asymmetrically bonded to the bridging carbonyls. However, within 0.1 kcal/mol lies a C 2 symmetry structure with one symmetric and two asymmetric bridging carbonyls. Furthermore, the high symmetry D 3 h structure analogous to Fe2(CO)9 lies only ∼1 kcal/mol higher in energy. The Cr2(CO)9 molecule is thus highly fluxional. The extremely flat potential energy surface in the region adjacent to these minima suggests that Cr2(CO)9 will be labile. The relationship between the Cr2(CO)9 molecule and the experimentally known binuclear manganese (η5-Me5C5)2Mn2(μ-CO)3 compound is explored.

Objectives Early age at natural menopause has been associated with increased all-cause mortality in several studies, although the literature is not consistent. This relation has not been examined among African American women. Study design Data were from the Black Women’s Health Study, a follow-up study of African-American women enrolled in 1995. Among 11,212 women who were naturally menopausal at entry to the study or during follow-up through 2008, we assessed the relation of age at natural menopause to all-cause and cause-specific mortality. At baseline and biennially, participants reported on reproductive and medical history, including gynecologic surgeries and exogenous hormone use. Mortality data were obtained from the National Death Index. Multivariable Cox proportional hazard models were used to estimate mortality rate ratios (MRR) and 95% confidence intervals (CI) for categories of age at menopause. Results Of 692 deaths identified during 91,829 person years of follow-up, 261 were due to cancer, 199 to cardiovascular diseases and 232 to other causes. Natural menopause before age 40 was associated with increased all-cause mortality (MRR=1.34, 95% CI 0.96-1.84, relative to menopause at 50-54 years; P-trend=0.04) and with the subcategories of death considered--cancer, cardiovascular disease, and all other causes. The associations were present among never and ever users of postmenopausal female hormones and among never and ever smokers. Conclusions In this large prospective cohort of African-American women, natural menopause before age 40 was associated with a higher rate of all-cause and cause-specific mortality. These findings provide support for the theory that natural menopause before age 40 may be a marker of accelerated somatic aging.