Hi Colin, you can add f' for every atom type in SHELXL yourself, so in that sense, it has been incorporated in SHELX. Bear in mind that the nucleus is point-like to X-rays at ordinary wavelengths so that it should not have a form factor like the electron cloud but a constant scattering length - just as they do for neutron scattering.
You can do the maths at what resolution the form factor and the constant 1:1860 scattering length contribution cross. It is not ridiculously small but nowhere near 0.8A. Charge density people may need to take this into account, but I don't know if they do. Cheers, Tim On 02/02/2015 04:03 PM, Colin Nave wrote: > “As you say the proton itself is invisible to X-rays.” > Not quite! The ratio of scattering between electrons and protons should go as > the inverse square of the masses. > Ratio of mass 1:1860, ratio of scattering 1:3459600. A small correction but > doubtless it has been incorporated in to SHELX. > Colin > > > From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Ian > Tickle > Sent: 02 February 2015 13:35 > To: ccp4bb > Subject: Re: [ccp4bb] proton scattering by X-rays > > > Peter, if it's a covalently-bonded H atom it surely can't be a bare proton, > it must have at least some partial electron around it for the (possibly > partial) covalent bond, enough to diffract X-rays anyway. As you say the > proton itself is invisible to X-rays. > Cheers > -- Ian > > On 2 February 2015 at 13:08, Peter Moody > <pcem1bigfi...@gmail.com<mailto:pcem1bigfi...@gmail.com>> wrote: > Dear BB > > I have (again) realised how limited by understanding of our subject is. > > In Nature’s online site > http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14110.html?WT.ec_id=NATURE-20150129 > there is a paper describing an X-ray structure determined with sub-atomic > data (nice!). The figures show density for H+ as well as H-. In my simple > way I had assumed that any X-ray scattering from the nucleus was negligible, > and that the electrons are responsible for this. I would expect a proton > (i.e. H+) alone to be invisible to X-rays, and certainly not to look similar > to a hydride (with two electrons in (electron density) maps. What have I > missed? Could someone please explain, or point me to a suitable reference? > > Best wishes, Peter > (please use peter.mo...@le.ac.uk<mailto:peter.mo...@le.ac.uk> to reply > directly) > > http://www2.le.ac.uk/departments/biochemistry/staff/moody > > > -- Dr Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A
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