Highly Strained Pn(CH)3 (Pn = N, P, As, Sb, Bi) Tetrahedranes: Theoretical Characterization.

Recent experimental research by Cummins and co-workers has established the existence of a tetrahedrane molecule with one CH moiety replaced by phosphorus. We present here the first theoretical studies of the entire Pn(CH)3 (Pn = N, P, As, Sb, Bi) class of molecules. Geometries are obtained at the highly reliable CCSD(T)/aug-cc-pwCVTZ(-PP) level of theory. Harmonic vibrational frequencies are determined and analyzed to confirm the nature of each stationary point and provide helpful findings that may aid in the detection of each species. Most notable is the result that the geometric parameters associated with the (CH)3 moiety in the tetrahedranes exhibit little change under pnictogen substitution, while the Pn-C bonds and C-Pn-C bond angles greatly increase and decrease, respectively. Strain energies are predicted and range from 122.3 kcal mol-1 (N(CH)3) to 99.4 kcal mol-1 (Bi(CH)3) at the DF-CCSD(T)//B3LYP-D3/aug-cc-pV(T+d)Z(-PP) level of theory. The obtained geometries are further analyzed with Natural Bond Orbital (NBO) methods to understand the bonding and electronic structure of each species. We also provide insight into how different substituents can help make the tetrahedrane structure more energetically favorable due to electron delocalization into substituent antibonding orbitals. The effect of additional delocalization also weakens the Pn-C bonds, especially for the heavier pnictogens. This work concludes with a list of considerations that summarize our key findings and motivate future work aimed at producing novel pnictogen-substituted tetrahedrane molecules.

Repeated hematocrit measurements in low-risk pregnant women.

OBJECTIVE: To determine the incidence of anemia in the third trimester among women with normal hematocrits early in pregnancy. STUDY DESIGN: Low-risk women seeking prenatal care were identified. Patients with a hematocrit >33% in the first or second trimester and a hematocrit drawn in the third trimester were included. Rates of anemia; hematocrit <33%; and severe anemia, hematocrit < 30%; were determined. The primary outcome was the development of anemia in the third trimester. RESULTS: A total of 1,604 women met inclusion criteria; 16.2% of women became anemic, and 4.5% became severely anemic in the third trimester. Using a starting hematocrit value of 39% as a cutoff, only 66% of women would need to be rescreened in the third trimester in order to detect 78.5% of anemias (sensitivity 78.5%, specificity 39.7%, negative predictive value 90.5%). CONCLUSION: The percent of women with normal hematocrits in the first trimester who become anemic in the third trimester is 16.2. Hematocrit screening in the third trimester may be unnecessary for low-risk women with starting hematocrits > 39%.

Organizing to learn: recognizing and cultivating learning communities.

One of the most effective ways to learn is in a community of people with a shared purpose. Therefore, by recognizing and cultivating the learning communities that arise within most organizational structures, we are actually organizing ourselves to learn. Using stories to inspire the reader to reflect and apply these concepts, the author explores learning communities in a variety of fields, including examples that involve entire organizations, single workgroups, and those that cross organizational boundaries.