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:[log in to unmask]] 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 <[log in to unmask]<mailto:[log in to unmask]>> 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 [log in to unmask]<mailto:[log in to unmask]> 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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