Oxirene, the most notorious case of an unsolved computational “existence
theorem”, has been reviewed (references to 2003) [ 10 ].
9.1.1.2 Nitrogen Pentafluoride. Unwarranted Optimism?
Nitrogen pentafluoride represents an interesting contrast to oxirene. Oxirene is, on
paper, a reasonable molecule; there is no obvious reason why, however unstable
it might be because of antiaromaticity [ 11 ] or strain [ 12 ], it should not be able to
exist. On the other hand, NF 5 defies the hallowed octet rule; why should it be more
reasonable than, say, CH 6? Yet a comprehensive computational study of this
molecule by Bettinger et al. left “little doubt” that it is a (relative) minimum
on its potential energy surface [ 13 ]. The full armamentarium of post-HF ab initio
methods, CASSCF, MRCI, CCSDT, CCSD(T), MP2 (Section 5.4) and DFT
(Chapter 7) was employed here, and all agreed that D3h(Section 2.6) NF 5 is a
minimum. Nevertheless, it was unclear that this paper (1998) fully disposed of
earlier (1989–1992) reservations about the ability of nitrogen to bear five fluorines:
Christe and coworkers concluded that “the lack of pentacoordinated nitrogen species
is due mainly to steric reasons”, from their finding that attack of HF 2 "(evidently a
surrogate for F") on NF 4 +occurs, within experimental error, only on F and not on
N[ 14 ]. This experiment dampened, but did not negate, hope arising from ab initio
computations by Ewig and Van Wazer indicating that NF 5 [ 15 ] and even NF 6 "[ 16 ]
may be able to exist. Comments by Christe and by Van Wazer and Ewig in letters to
C&EN [ 17 ] showed that each was at the time unpersuaded by the position of the
other. Reinforcing their studies in connection with NF 4 F[ 14 ], Christe and Wilson
were led by experiment and theoretical arguments to conclude that “covalent NF 5
should suffer from severe ligand-crowding effects that would make its synthesis
very difficult” [ 18 ]. The difficulty of accurately accounting computationally for
crowding around a central atom [ 19 ] was evidently the reason for doubts^1 about
the possibility of making nitrogen pentafluoride, but these reservations have been
overcome, evidently by reconsideration of the work of Bettinger et al.^2
9.1.1.3 Pyramidane. A Realistic Goal
If oxirene “should” exist and NF 5 “should” not, what are we to make of pyramidane
(Fig.9.3)? This molecule contradicts the traditional paradigm [ 20 ] of tetracoordi-
nate carbon having its bonds tetrahedrally directed: the four bonds of the apical
carbon point toward the base of a pyramid. Part of the calculated [21] potential
energy surface of pyramidane is shown in Fig.9.3. To improve the accuracy of the
(^1) Personal communication from Professor Christe, 2007 April 24.
(^2) Personal communication from Professor Christe, 2010 April 16; he concludes that NF
5 can exist,
although “the synthesis would be difficult”.
564 9 Selected Literature Highlights, Books, Websites, Software and Hardware