= Dipole evidence for bond polarization
A permanent <electric dipole moment> gives evidence of separated positive and negative <electric charge> in an overall neutral <molecule>. This is a more direct probe of <chemical polarity> than an <electronegativity> table. For <hydrogen chloride>, NIST lists about $1.109$ <debye> for the lowest vibrational state and $1.093$ <debye> for the equilibrium value. They describe different averaging conventions, not two contradictory measurements. The NIST WebBook traces the vibrational value $1.1085$ <debye> to molecular-beam electric-resonance measurements in 1970.<HCl (Hydrogen chloride): experimental data by National Institute of Standards and Technology>{ref}<Hydrogen chloride: Constants of diatomic molecules by Klaus P. Huber and others>{ref}
How does <molecular beam electric resonance> reveal an <electric dipole moment>? A source and apertures form a beam of <molecules> in <vacuum>. A first nonuniform <electric field>, A, selects <quantum states> through their different <Stark effects>. In a central region, C, electrodes supply a nearly uniform <electric field> and applied radiation drives transitions at selected <frequencies>. A second nonuniform <electric field>, B, analyzes the outgoing <quantum states>; a detector measures the rate of arriving <molecules>. In a transmission-loss arrangement, resonant radiation transfers some <molecules> to a <quantum state> rejected by B, so the measured arrival rate dips. The observable is a change in detected <molecules> as the radiation <frequency> is scanned, rather than light absorption itself. This general apparatus description is documented in a later experimental thesis; it is not a reconstruction of the exact 1970 HCl instrument.<High Resolution Molecular Beam Study of Alkali Cyanides by Johannes Jacobus van Vaals>[§2, printed pp. 11–14: fields A, C and B, detected flux, parallel plates, radiation and detector]{ref}
\Image[/bond-dipole-beam-resonance.png]
{title=<Molecular beam electric resonance>: schematic state selection and a synthetic <Stark effect> signal from a <rigid rotor> model; neither apparatus dimensions nor signal curves are measured HCl data}
{id=bond-dipole-beam-resonance}
The inference comes from how transition <frequencies> change with the known <electric field>. The interaction $-\boldsymbol\mu\cdot\mathbf E$ couples the <electric dipole moment> to the <electric field>; fitting the <quantum mechanics> of the relevant rotational states extracts its magnitude. A free rotating <molecule> need not show a first-order shift proportional to $|\mathbf E|$: the weak-field <Stark effect> can be quadratic. The diagram uses a simplified <rigid rotor>, with $B$ an <energy> constant, to illustrate this behavior. It omits the fine details of actual spectra.
Useful controls include scans with the radiation off to measure the baseline arrival rate, repeated scans at several calibrated <electric fields>, and agreement between independently assigned transitions. These are methodological checks, not claims about undocumented steps of the 1970 experiment. Field calibration and state assignment limit the inferred <electric dipole moment>. The published 1970 abstract confirms analysis of HCl and DCl beam-resonance spectra and combination with <infrared spectroscopy> intensities to obtain a distance-dependent <electric dipole moment>; its full apparatus and calibration record were unavailable here.<Dipole Moment and Hyperfine Parameters of H35Cl and D35Cl by E. W. Kaiser>[pp. 1686–1703, authored abstract; full text unavailable]{ref}
Even an accurately measured <electric dipole moment> is not an absolute <partial atomic charge>. Replacing a <hydrogen chloride> <molecule> by two point <electric charges> $\pm q$ separated by <bond length> $r$ gives $|\mu|=|q|r$. Using $1.109$ <debye> and $r=1.27455\times10^{-10}\,\mathrm m$ gives $|q|\approx0.181$ times the elementary <electric charge>. This is an effective two-point model, combining a vibrationally averaged <electric dipole moment> with an equilibrium <bond length>, not a unique partition of the actual <electron density> or a measured “18.1 percent ionic bond”.<Hydrogen chloride: Constants of diatomic molecules by Klaus P. Huber and others>{ref}
For a <molecule> with several <chemical bonds>, <molecular geometry> can cancel opposing contributions to the total <electric dipole moment>. <Carbon dioxide> illustrates why an overall <nonpolar molecule> need not have unpolarized individual <chemical bonds>. An <electronegativity> comparison helps anticipate local <bond polarization>; the measured molecular <electric dipole moment> tests the entire <electric charge> distribution.
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