The electronic orbitals possible for an AH, molecule when linear are correlated with those possible for the molecule when non-linear. Qualitative curves of binding energy versus LHAH are given for the various orbitals. These curves are then used to explain and predict (i) the shapes and (ii) the electronic spectra and associated characteristics of AH, molecules. AH, molecules containing 4 valency electrons should be linear in their ground states. AH, molecules containing 5-8 valency electrons should be bent in their ground states. THE purpose of this paper is first to correlate the electronic orbitals of a bent with a linear AH, molecule, next to decide whether a given orbital becomes more or less tightly bound with increase of the angle HAH, and then to use the resulting graphs of binding energy versus apex angle to explain and predict the shapes and spectra of AH, molecules.Linear AH, MoZeczcZes.T-The lowest-energy intra-valency-shell orbitals of a linear AH, molecule, may, on the assumption that these are built solely from s and fi atomic orbitals, be described as follows: (i) Two orbitals binding the hydrogen atoms to the central atom. These may be thought of as each formed by the overlap of an sfi hybrid atomic orbital on A with the 1s orbital of hydrogen. If so regarded they will be orbitals of a-type predominantly localized one in each A-H distance. If, however, one were discussing a transition involving excitation of an electron from one of the bonding orbitals, one could not regard the electron as coming from one A-H link or the other-the two are indistinguishable. If one first conceives of the orbitals as completely localized one has to take combinations of them in order to express this indistinguishability. These combinations are either in-phase or out-of-phase and may be labelled og and 6, respectively.(ii) A z,~ orbital. This is simply a 9 orbital localized on the central atom and pointing in a direction at 90" to the HAH line. It is non-bonding. Since there are two such directions that are independent but equivalent (except for a rotation by go"), the orbital is two-fold
A correlation diagram is plotted for the orbitals of linear and non-linear triatomic molecules. Simple reasons are given why particular orbitals become more or less tightly bound as the apex angle changes. The diagram is used to interpret and predict the shapes, reactivities, and spectra of nonhydride triatomic molecules. As regards the shapes, the following facts (most of which have been recognised for some time) become understandable : that molecules with not more than 16 valency electrons are linear in their ground states; that molecules with 17, 18, 19, or 20 valency electrons are bent in their ground states, the apex angle decreasing markedly from 16-to 17-and from 17-to 18-electron molecules and less markedly from 18-to 19and from 19-to 20-electron molecules; and that 22-electron molecules are linear or nearly linear in their ground states. The paper has much in common with an earlier paper by Mulliken. THE purpose of this paper is to apply to AB, and BAC molecules the procedure which in Part I * was applied to AH, molecules. We consider first symmetrical, non-hydride, triatomic molecules AB,.AB, Molecules.-The lowest-energy orbitals of a linear AB, molecule may, on the assumption that they are built solely from s and p atomic orbitals, be described as follows :(i) Two lone-pair s orbitals, one on each B atom. (ii) Two bond orbitals analogous to the bond orbitals discussed in Part I. For discussion of molecular shape, they may be thought of as built from sp hybrid valencies of A plus $ 0 valencies of the two B atoms and largely localized one to each A-B distance. In discussion of spectroscopic transitions involving them, they would have to be regarded as non-localized combinations of the localized orbitals and be labelled og and ou. (For simplicity, hybridization of the s and the * Even in liquid CO, no absorption occurs until a t least 2150 A. 7 It is easily seen that in N,O the analogue of the lower .rr, orbital is predominantly localized on the 0 atom, while the analogue of the wv orbital is predominantly localized on the end N atom. * * -(a2")(")2(a1')2(b1''), 1B2 t--* * (a,")2(b2')"(a1')2, 1A, . . This is an allowed transition that should give rise to parallel bands and lead to a moderate decrease of apex angle and increase of S-O length. ' * ' ( ~~" ) 2 ( ~~' ) ( ~1 ' ) 2 ( b l ' ' ) , * * -(b2')~a2'')2(b~')2(a,')2(b1''), ' A , +-* * -(b2')2(a2'')2(b,')2(a1')2, 'Al . (23) This is a forbidden transition. * * -( ~~~' ) ( b ~" ~( a ~" ) ~( b ~' ) ~( a ~' ) ~( b ~' ' ) , ' B , +-* * * (al')2(~2')2(~2'')2(b2')2(~1')2, 'Al .
New data on the ultra-violet absorption spectrum of ammonia and deuterated ammonias are reported and used, together with older data, in an interpretation of the spectrum. Five electronic transitions are particularly studied. Each is represented by a long progression of bands. The origins of four of the transitions lie at 2168 A, 1665 A, 1434 8, and 1330 8, ; the fifth is of unknown origin but is responsible for bands in the neighbourhood of 12688,. The first four transitions are each proved, by a vibrational analysis, to have planar upper states. It is concluded that the upper state of the fifth and the ground state of the NH; ion are also planar. The upper state of the 2168A transition is shown to have A; electronic symmetry and an N-H length greater than in the ground state. Some extra information concerning the ground-state vibrational levels is deduced. All the transitions studied are Rydberg in type, though the bands of the 2168 8, transition are predissociated through the influence of the lowest energy singlet excited intra-valency shell state ; the transitions lead to an ionization limit at 10.18 eV in good agreement with a photon impact value. Electronic and vibronic assignments of all the transitions are made. The v2A;I frequency in the ground state of NH$ is 920&10 cm-1.
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