Hi Colin, I understood the jest, of course.
Now I got curious: At 2theta=0, the scattering from H is 1e, so I assume the the scattering length for its nucleus is 1e/1860 = 0.00054. According to the data from Phil Coppens web site, the atomic scattering factor for H reaches this value at sin theta / lambda = 1.95, i.e. d=0.26A. That's far away from the wavelength 'we' use, but not too far off from the resolution limit on a Silver source (0.31A), is it? I am not sure this can be totally neglected. Am I wrong? Cheers, Tim On 02/03/2015 04:03 PM, Colin Nave wrote: > Hi Tim > Although my SHELX comment was in jest, your point illustrates the programs > versatility. You are also right about the flat(ish) form factor for the > proton. > To get to a resolution where there is a cross over would require a very short > wavelength. Other processes would then dominate. A nice source for this is > the x-ray data booklet from LBL, in particular the chapter on scattering of > x-rays from electrons and atoms. > http://xdb.lbl.gov/Section3/Sec_3-1.html > Interestingly fig 3-1 in this does not include coherent scattering from > nuclei presumably because it is negligible compared with the other processes > - in practice Ian was correct in saying that a proton is effectively > invisible to x-rays of the energy we usually use. > > Colin > > > -----Original Message----- > From: Tim Gruene [mailto:t...@shelx.uni-ac.gwdg.de] > Sent: 02 February 2015 22:08 > To: Nave, Colin (DLSLtd,RAL,LSCI); ccp4bb > Subject: Re: [ccp4bb] proton scattering by X-rays > > 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 > > -- Dr Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen GPG Key ID = A46BEE1A
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