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TiC2 crystallizes in the orthorhombic Amm2 space group. The structure is three-dimensional. Ti4+ is bonded in a 8-coordinate geometry to eight equivalent C2- atoms. There are four shorter (2.20 Å) and four longer (2.32 Å) Ti–C bond lengths. C2- is bonded in a 5-coordinate geometry to four equivalent Ti4+ and one C2- atom. The C–C bond length is 1.34 Å.
TiC(NF2)3CN3 crystallizes in the monoclinic Cm space group. The structure is two-dimensional and consists of two guanidine molecules and one TiC(NF2)3 sheet oriented in the (0, 0, 1) direction. In the TiC(NF2)3 sheet, Ti4+ is bonded in an octahedral geometry to six F1- atoms. There is two shorter (1.89 Å) and four longer (1.90 Å) Ti–F bond length. C4+ is bonded in a trigonal planar geometry to three N1- atoms. There is one shorter (1.34 Å) and two longer (1.35 Å) C–N bond length. There are two inequivalent N1- sites. In the first N1- site, N1- is bonded in a distorted single-bond geometry to one C4+ atom. In the second N1- site, N1- is bonded in a distorted single-bond geometry to one C4+ and two F1- atoms. There are one shorter (2.43 Å) and one longer (2.56 Å) N–F bond lengths. There are four inequivalent F1- sites. In the first F1- site, F1- is bonded in a distorted single-bond geometry to one Ti4+ and two equivalent N1- atoms. In the second F1- site, F1- is bonded in a distorted single-bond geometry to one Ti4+ atom. In the third F1- site, F1- is bonded in a distorted single-bond geometry to one Ti4+ atom. In the fourth F1- site, F1- is bonded in a distorted single-bond geometry to one Ti4+ and one N1- atom.
In the Arctic, during polar night and early spring, ice clouds are separated into two leading types of ice clouds (TICs): (1) TIC1 clouds characterized by a large concentration of very small crystals and TIC2 clouds characterized by a low concentration of large ice crystals. Using a suitable parameterization of heterogeneous ice nucleation is essential for properly representing ice clouds in meteorological and climate models and subsequently understanding their interactions with aerosols and radiation. Here, we describe a new parameterization for ice crystal formation by heterogeneous nucleation in water-subsaturated conditions coupled to aerosol chemistry in the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). The parameterization is implemented in the Milbrandt and Yau (2005a, b) two-moment cloud microphysics scheme, and we assess how the WRF-Chem model responds to the run-time interaction between chemistry and the new parameterization. Well-documented reference cases provided us with in situ data from the spring 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) over Alaska. Our analysis reveals that the new parameterization clearly improves the representation of the ice water content (IWC) in polluted or unpolluted air masses and shows the poor performance of the reference parameterization in representing ice clouds with low IWC. The new parameterization is able to represent TIC1 and TIC2 microphysical characteristics at the top of the clouds, where heterogenous ice nucleation is most likely occurring, even with the known bias of simulated aerosols by WRF-Chem over the Arctic.