Isotope substitution — where it appears
Named by 15 essays across 2 fields — each of them below, with the objects they name alongside it.
When a mode becomes a bond stretch
Water's two stretching modes are each exactly half in one O–H bond and half in the other, which is why neither of them belongs to a bond. Change one hydrogen to deuterium and the same force field at the same geometry gives two modes that are 99.5 and 99.7 per cent in a single bond each. Nothing about the bonding changed; a mass did.
Three numbers is not a structure
Formaldehyde's rotational spectrum gives three constants, of which a planar molecule's are only two independent numbers, and its structure has three parameters. Seven structures are computed here that reproduce A and B to the last digit the solver carries: the C=O length runs from 1.000 to 1.300 ångström, the C–H length from 1.557 down to 0.952, and the HCH angle from 74.6 degrees to 164.5.
The coordinate an isotope reports
Kraitchman's equations return an atom's position from the change in the moments when that atom alone is made heavier, and for a rigid structure they are an identity — formaldehyde's four atoms come back to a part in ten million. What they return is the square of each coordinate, so both hydrogens at b = ±0.9348 come back at +0.9348; every out-of-plane coordinate comes back imaginary at a moment error of one part in a hundred thousand; and the famous error cancellation, measured at a factor of thirteen, still leaves the answer two and a half times worse than a direct fit.
The correction that was invented
A standard error model puts the zero-point error in a rotational constant at a few tenths of a per cent, shared between the three moments by invented weights, with the parent and its deuterated form differing by five. Computed from a force field it is 1.88 per cent, one of the three shares is negative, and the mismatch is 35 — so the cancellation the substitution method rests on is worth a factor of 2.5 and not thirteen.
The axis that goes the other way
One dimensionless coefficient turned a zero-point correction into an expression a spectroscopist could evaluate, and the three principal axes were averaged over to get it. Split by axis it gets worse, not better — the coefficients span 2.3 where the molecule-averaged ones span 1.36 — and water's smallest moment does not grow at all. It shrinks.
Two moments about two different lines
Asked axis by axis, the substitution method's near-cancellation gives numbers as large as 163 per cent. The arithmetic is the smaller half of the answer. A principal axis is an eigenvector of a tensor built from the masses, so one deuterium turns water's frame by 21.12° — and the two moments being compared are not moments about the same line.
The term a harmonic field cannot produce
The usual zero-point correction to a moment of inertia comes from a harmonic force field, which contains the mean square displacement and nothing else. A rotational constant does not average that. It averages one over r squared, whose leading correction is the mean displacement — zero in any symmetric well — and which enters with the opposite sign and about twice the size.
The cubic a Morse curve guesses
The two terms in a vibrationally averaged rotational constant were computed on Morse curves built from measured constants, and the vibration–rotation constant αₑ is the measurement that tests them. In all four molecules the Morse curve's αₑ is short, by four to fifteen per cent. The averaging is not the error — it matches the closed form to four parts in ten thousand. The curve's cubic is, and the measurement asks for more of exactly the term a harmonic field cannot produce.
Adding data made it worse
A projection leaves a residual spread across four starting points, and an earlier essay attributed it to the eigenvalue problem rather than to the coordinate set — an attribution rather than a measurement. Refitting with two isotopologues was the proposed test. Run, it produces a runaway to minus ninety-seven thousand that one isotopologue alone does not, and the reason is that the direction it walks along is flat at every mass.
The residual was a loop
Projecting away boron trifluoride's flat direction left four fits disagreeing by a tenth in the B–F stretch constant, and the disagreement was put down to the search or to the arithmetic. It is neither. Two frequencies cannot fix three constants in one symmetry block, so the exact fits form a closed curve along which the stretch constant runs from 3.9 to 12.1, and the four fits are four points on a short arc of it.
The third mass that cannot help
Boron trifluoride's E′ block has three force constants and two frequencies, so its exact fits form a closed curve, and a boron-10 substitution cuts that curve to two fields. The obvious remedy is a third isotopologue, and it was named as the first data that could pick one of the two. It is not, if the third substitution keeps the molecule's symmetry: every such isotopologue gives the same spectrum under both fields to a millionth of a wavenumber, because its sum rule is one plane written in two reciprocal masses and its product rule is Teller and Redlich's. Only a substitution that breaks the symmetry adds an equation — and it separates the two fields by half a wavenumber.
The worst case was the loosest constant
A symmetry label's verdict on a distortion is read off mode shapes, and mode shapes are set by a fitted force field. Walked along the direction its own frequencies see least, ammonia's field moves the verdict on its bond angle from 0.840 to 0.923 without the fit getting measurably worse — and boron trifluoride, the census's worst case at 0.573, has a whole curve of fields that reproduce its spectrum exactly, along which the same verdict runs from a coin to 0.862.
The fourth constant chose a curve, not a depth
Read from its lowest four levels the way every potential here is read, carbon monoxide's measured ωₑyₑ is 0.01101 cm⁻¹ — and of the two bent-range Morse curves that fit its first three constants, the one that predicts 0.01100 is the one that never dissociates. Let the range bend and stay bounded and every molecule, the hydrides included, is fitted to all four constants by a curve that does dissociate. The depths those curves carry are 6 to 31 per cent too deep, further from the measurement than the plain Morse curve's on all four, and hydrogen chloride and deuterium chloride — one potential — are fitted by two different ones.
Two Coriolis constants are one number
Ammonia's frequencies leave its force field free along a band, and across that band a symmetry label's verdict on the bond angle changes. The measurement proposed to settle it was the Coriolis constant of each E vibration, which reads mode shapes directly — and it would settle it, sixteen times more sharply than the frequencies do. But the two constants are not two measurements. Their sum is fixed by the geometry and the masses, ζ₃ + ζ₄ = I∥/2I⊥ − 1 (the moments of inertia about the axis and across it), on every field there is, so the spectrum's Coriolis constants carry exactly one number about the field between them.
The Coriolis constant was an isotope shift
Boron trifluoride's E′ block has a closed loop of force fields that fit its spectrum exactly, a ¹⁰B spectrum cuts the loop to two, and no third isotopologue that keeps the symmetry can choose between them. The Coriolis constant depends on mode shapes rather than frequencies, so it looked like the measurement that could. Around the loop it takes every value from −1 to +1 — and under the two fields it is the same to nine decimals, because for a planar top it is exactly the boron's share of the motion, which is what a boron isotope shift measures.
Named alongside it
The objects these essays reach for when they reach for this one.
Force constantModel limitNormal modeConventionRotational constantDegeneracyLeast-squaresMoment of inertiaUnderdeterminationValence force fieldZero-point energyIrreducible representations