Additions and Corrections
1470 The Journol of Physical Chemistry, Vol. 97, No. 7, 1993
ADDITIONS AND CORRECTIONS
1992, Volume 96 Alexander I. Boldyrev a d Jack Simons’: Rydberg Bonding in (NH4)2* Page 8841. We stated “We believe the appearance of this imaginary frequency is an artifact that is caused by second-order instability in the electronic wave function mentioned in the preceding paragraph as well as by using numerical finitedifference techniques for calculating second derivatives in the MPS(fu1l) case.” We have found this statement to be incorrect. In subsequentcalculations,using the new Gaussian82 program, we first computed vibrational frequencies using onolyticol second derivatives at the MP2(fu11)/6-31++G** level and found that the imaginary values v16(e”) = 109.8ipersist for the D3h (IAl”) structure. We therefore reoptimized the geometry of (NH4)2 with only a C2 symmetry constraint and found a true local minimum of C2h (IA8) symmetry whose structure is shown in Figure 1. (The optimized bond lengths and angles are R(NIN2) 3.633 A, R(Nl-H3) = 1.035 A, R(N1-H.J R(NI-Hs) = 1.042A,R(Nl-Hs) = 1.035A, LH3NlH4 = LH3NlHs = 111.1’, LH3NlH6 111S0, LH4NlHs 107.0’, LH4NlH6 = 108.0’, and L H ~ N ~= N 159.6O.) ~ All frequencies are positive for this structure, and the lowest frequency is greater than 50 cm-1 (see Table I). We also recalculated the total energies at all correlated levels: MP2, MP3, MP4, QCISD, and QCISD(T) with 6-31++G** basis sets and found them to be 0.2-0.3 kcal/mol lower at the new C2h (]Ag)geometry than at the earlier D3h (1Al’) geometry. Further optimization of this molecule’s structure (with the C2
Figure 1. Minimum-energy CZh structure of (NH4)Z.
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restriction) at the MP3/6-31++GS* level (R(NI-N2) = 3.699 A, R(NI-H3) = 1.033 A, R(Nl-H4) R(NI-Hs) 1.038 A, R(Nl-Ha) = 1.032A, LH3NlH4 = L H ~ N I H 1~1 1.0°,LH3NlH6 = 111.4°,LH4NlHS=107.1°,LH4N~H6= 108.1’,andH3NIN2 = 159.2O)leads to the some C2h (IA,) structure and a total energy only 0.07 kcal/mol lower than at the MP2(fu11)/6-3 1++G** geometry. The very small corrections to the total energy of (NH4)2 reported here do not change significantly our earlier dissociation energies into NH4 + NH4 or into NH3 + NH3 + H2, nor do they affect our prediction of the ability of Rydberg orbitals to form weak chemical bonds. They have absolutely no effect on our predictions for (NH4)2+. In future publications we will consider (NH4)2 in more detail, as well as other molecules we have studied recently that seem to “avoid” high symmetry structures when treated at correlated levels although they have high symmetry structures at the SCF level. Finally, we also wish to note that the (NH4)2+ system had earlier been studied (Kassab, E.; Fouquet, J.; Evleth, E. M. Chem. Phys. Lett. 1988,153,522) as “ammoniated NHs+”. This earlier work and references therein to experimental references are important to note.
TABLE I: Calculated Harmonic Frequencies (em-’) and IR CU (‘AI) Structure of ( m ) at ~ sym
freq
int
3321 3251 3014 1656 1398 1382 151 105 3091 1635 1376 153
0 0 0 0 0 0 0 0 387 87 0 3
sym
frw
int ~~
Q 1993 American Chemical Society
56 3141 1648 1374 202 3334 3253 3042 1638 1439 1383 82
1 0 0 0 0 150 918 3189 150 21 1 93 0
