Molecular Geometry & Bonding
what the shape decides
Concept

Inversion splitting — where it appears

The interval between the two members of a tunnelling doublet, measured directly in a microwave or far-infrared spectrum. Ammonia's ground-state value is 0.79350 wavenumbers; it fixes an inversion barrier far better than a barrier predicts it, because the dependence is a steep power.

Named by 8 essays across one field — each of them below, with the objects they name alongside it.

NH₃: four states in a well the molecule does not sit at the bottom of. The umbrella coordinate of NH₃ — the signed distance of the N atom from the plane of its three H atoms — with the quartic well that has its minima at the measured 0.3816 ångström and its barrier at the quoted 2020 wavenumbers. The lowest 4 states are drawn at their computed energies. The lowest sits 587 wavenumbers above the bottom, which is 29.1 per cent of the way up the barrier, so the state is far from the harmonic bottom that a drawing of a pyramid implies.

A barrier is not what a splitting measures

Ammonia's inversion barrier is quoted everywhere as 2020 wavenumbers. Put that number into the simplest double well its own measured geometry allows and the ground-state splitting comes out at 1.3508 against a measured 0.7935, and the excited one at 68.37 against 35.81. Both are too large because a splitting is an area under a barrier and a height is only one of its two dimensions.

shape · Inversion One per cent on the barrier is 3.6 per cent on the splitting. The ground inversion splitting of NH₃'s quartic well against the barrier height, both logarithmic, with the geometry and the reduced mass held at their measured values. The curve is visibly bent: its local slope is -3.56 at the published barrier and steepens either side, so a power law is a tangent to it rather than a description of it. The measured 0.7935 wavenumbers is reached at 2330, which is 15.3 per cent above the quoted 2020 — so a splitting wrong by a factor of 1.70 is a barrier wrong by a sixth. The same derivative read the other way is what makes a barrier quoted to ten per cent useless for predicting a splitting.

The exponent that runs both ways

How hard does a splitting depend on a barrier? Locally, as the power −3.5628 — and the local slope runs from −2.53 to −6.15 across the same sweep, so there is no power law. What is exact is stranger: rescaling the equation forces the mass exponent to be one below the barrier's and the geometry exponent to be twice the mass's, so the model's three sensitivities are one number and the arithmetic reproduces both identities to six decimals.

shape · Inversion The mass is worth a factor of 1.6, and the other two 1e+4 and 9e+4. Ammonia's umbrella well, with each of phosphine's three differences substituted into it one at a time and then all together. The reduced mass is 11 per cent larger and costs a factor of 1.61. The pyramid is 2.01 times taller and costs 9.6e+3; the barrier is 6.1 times higher and costs 9.4e+4. Phosphine's own splitting is below what the arithmetic resolves, so it is drawn at that bound.

It was never the mass

Phosphine does not invert, and the reason given is that phosphorus is heavier than nitrogen. Three things about phosphine differ from ammonia. Substituting each into ammonia's own well one at a time, the reduced mass costs a factor of 1.61, the pyramid height costs 9,600 and the barrier 94,000 — and the mass is the smallest of the three by four orders of magnitude.

shape · Inversion Three constructions of one number, spanning a factor of 5.6. The reduced mass of NH₃'s umbrella coordinate under each construction, against position along the coordinate. Two of the three are constants and the third is not: if the bonds are held at their measured length, the ligands must slide outward as the apex descends, and their radial motion adds to the mass. It runs from 2.4866 at the plane to 2.9874 at the pyramid — 20 per cent — and the coordinate itself stops existing at one bond length, which is where the curve ends.

The mass nobody chose

Every one-dimensional treatment of ammonia's inversion needs a mass, and the measurement does not supply one. Three constructions are defensible and they give 1.35075, 0.94420 and 0.0000055 wavenumbers. The honest one is not a constant at all, and it moves the answer thirty per cent towards the measurement — which means the usual choice is the wrong one.

shape · Inversion The orderings in use move the splitting by 0.47 per cent between them. The change in NH₃'s ground inversion splitting under each ordering of the kinetic operator, relative to BenDaniel–Duke, with the bond-conserving mass throughout. The bars are exact solves and the ticks are first-order perturbation theory. The five span 0.469 per cent, from −0.407 to 0.060; the change from a constant mass to the bond-conserving one, in the same well and box, is 43.1 per cent, 92 times as large.

An ordering worth half a per cent

A mass that varies along a coordinate has no unique quantum kinetic energy, and the choice among the Hermitian orderings in use was the one thing left that could undo a forty-three per cent correction to ammonia's splitting. It cannot. The five orderings anybody uses span 0.47 per cent between them, a ninety-second of the correction, and the family only reaches the measurement at exponents three times larger than any of them.

shape · Inversion Every prediction of the isotope ratio overshoots, and the mass decides nothing. The ratio of NH₃'s ground inversion splitting to ND₃'s, predicted six ways, against the measured 14.94. At the published barrier the usual mass gives 15.77 and the bond-conserving mass 17.79. With a quartic fitted to NH₃'s splitting they give 19.26 and 19.00; with a well whose shape is fitted to both of NH₃'s lines, 17.62 and 18.48. The bond-conserving mass is nearer the measurement in one of the three pairs and further in two, and the difference within any pair is smaller than the distance of either from the measurement.

Deuterium cannot tell the masses apart

A reduced mass built by holding ammonia's bonds rigid predicts a deuterated molecule differently from any constant mass, and ND₃'s splitting is measured. Run as a test, it cannot choose. Every well and every mass needs a barrier for ND₃ several per cent lower than for NH₃, every prediction of the isotope ratio from a well fitted to NH₃ overshoots by eighteen to twenty-nine per cent, and the two masses differ by less than either misses — in opposite directions in the two wells.

shape · Inversion The term two of the four molecules have and two do not. The radial coefficient of the bond-conserving reduced mass for the four isotopologues: the sum of the ligand masses, and what is left after the asymmetric correction. Three ligands at a hundred and twenty degrees on a circle whose radius changes as the apex descends move their own horizontal centre of mass outward — unless the three masses are equal. A frame that does not translate has to subtract that motion, and the amount is half the sum of the squared mass differences over the total mass. It is zero at both ends of the series and the same number in the middle.

The two that are not on the line

Ammonia and its fully deuterated twin are two points, and two points cannot show a curve. Putting the partly deuterated molecules between them needs a term neither symmetric one has — three ligands of unequal mass move their own centre of mass sideways as the apex descends — and it moves the prediction by half a per cent, which is what a whole change of mass construction was worth.

shape · Inversion The covariant operator is BenDaniel–Duke plus this. The difference between the Laplace–Beltrami operator — the one a one-dimensional manifold with metric μ(x) distinguishes, carried across to the flat measure by the unitary map ψ ↦ μ^(¼)ψ — and the BenDaniel–Duke ordering, divided by the function it was applied to, at forty-one positions inside the molecule's own range. The curve drawn through the marks is the two-function fit every ordering is a combination of, and it passes through them to a part in ten million.

The ordering a manifold picks

A position-dependent mass leaves the kinetic energy with no unique quantum form, and an earlier sweep of the five orderings in use found half a per cent between them. A one-dimensional reduction is a one-dimensional manifold, a manifold has a distinguished Laplacian, and carrying it to the flat measure lands on exactly one of those five — not the one with no extra potential, and not the one anybody reaches for.

shape · Inversion

Named alongside it

The objects these essays reach for when they reach for this one.

Model limitReduced massDouble wellTunnellingUmbrella modeConventionIsotopologueZero-point energyBorn–Oppenheimer separationInternal coordinateBond angleClosed form

All concepts