Specific Heat Capacity of Heavy Water

Water has an unusually high specific heat capacity due to it's hydrogen bonds. 


Why is it that the change in isotope causes a 10% difference in the heat capacity? How does the added neutron in the nucleus make any difference to the bonds? 




I posted an answer here the other day which was completely wrong! I apologise and have left the incorrect text below for posterity to record how wrong people who should know better can be!


Looking on the NIST fluid properties database I see that the specific heat capacity (i.e. the heat capacity per unit mass) of D2O is just 1.5% higher than the specific heat capacity of H2O. So at 298 K this makes the molar heat capacity of D2O (84.963 J/K/mol) an astonishing 12.7% higher than H2O (75.38 J/K/mol). I am astounded and have no explanation. 


Everything below this line is WRONG!
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*The molar heat capacity is almost exactly the same. i.e. it takes the same energy to heat the same number of molecules by one degree Celsius. 
The difference in the \*specific\* heat is just because of the difference in density. D2O molecules have a relative molar mass of about 20 compared with 18 for H2O. Since the bond lengths are similar, the density is higher by a factor 20/18 - about 11% higher. So a given mass of D2O has about 11% fewer molecules in it.*


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